
Have you ever imagined a geometrical shape, and then had trouble describing it? Many shapes have names, but they're fairly obscure, so a glossary might help.
General notes: planes, lines, cones, and cylinders extend infinitely in both (or all) directions. A cone, in particular, is not shaped like an "ice cream cone" but rather like two such objects, attached at their pointy ends, and extending infinitely in both directions. Shapes that are bounded are called "line segments", "frustums", "solid cones", "prisms", etc. So read the definitions carefully. (See also thesaurus and mathworld)
acute angle  an angle whose measure is greater than 0 but less than 90 degrees
adjacent angles  2 nonstraight and nonzero angles that have a common side in the interior of the angle formed by the noncommon sides
alternate exterior angles  exterior angles on alternate sides of the transversal (not on the same parallel line)
alternate interior angles  interior angles on alternate sides of the transversal (not on the same parallel line)
angle  the union of 2 rays that have the same endpoint; measured in degrees or radians; the five types of angles are zero, acute, right, obtuse, and straight
angle bisector  a ray that is in the interior of an angle and forms two equal angles with the sides of that angle
annulus  a ring; the area bounded by two concentric circles
apothem  in a regular polygon, the perpendicular distance from the center to a side; in a circle with a chord, the distance from the midpoint of a chord to the circle's center

arbelos  a plane region bounded by three mutually tangent semicircles whose diameters are collinear.
arc  a continuous portion (as of a circle or ellipse) of a curved line
arc length  the distance between an arc's endpoints along the path of the circle
area  the surface enclosed within a closed plane figure; the measure of the surface, expressed in equivalent square units, such as square inches.
axis  a straight line about which a body or a geometric figure rotates or may be supposed to rotate; a straight line with respect to which a body or figure is symmetrical  called also axis of symmetry; one of the reference lines of a coordinate system
ball  a "solid sphere"; the interior of a sphere (open ball); a
sphere and its interior (closed ball);

Borromean rings  A set of three rings joined in such a way that no pair is interlinked, but the three cannot be separated.
bundle  the set of planes through a point, in projective geometry.
cage  regular graph that has as few vertices as possible for its girth. Formally, an (r,g)graph is defined to be a graph in which each vertex has exactly r neighbors, and in which the shortest cycle has length exactly g. The (r,3)cage is a complete graph K_{r+1} on r+1 vertices, and the (r,4)cage is a complete bipartite graph K_{r,r} on 2r vertices. Other cages listed in Wikipedia include the Moore graphs: (3,5)cage: Petersen graph, (3,6)cage: Heawood graph, (3,8)cage: Tutte�Coxeter graph, (7,5)cage: Hoffman�Singleton graph.
Cartesian plane  a coordinate plane
central angle of a circle  an angle whose vertex is the center of the circle
cevian  a line segment connecting a vetex of a triangle with a point on the opposite side of the triangle. The condition for three general Cevians from the three vertices of a triangle to concur (at a cevian point) is known as Ceva's theorem.
cevian point  the point of concurrence of three cevians
cevian triangle  Given a point P and a triangle ABC, the Cevian triangle A'B'C' is defined as the triangle composed of the endpoints of the cevians through the Cevian point P.
chord  In general, a straight line joining two points on a curve; often, chord is used to mean a straight line segment joining and included between two points on a circle;
chord of a circle  a segment whose endpoints are on a circle
circle  a closed plane curve every point of which is equidistant from a fixed point, called the center (the center is not part of the circle)
circular  having the shape of a circle; a circular cylinder is one in which its defining shape is a circle, and most often is used to mean a right circular cylinder; a circular cone is one in which its defining shape is a circle, and most often is used to mean a right circular cone;
complementary angles  2 angles whose measures, when added together, equal 90 degrees
complex polygon  a polygon having at least one pair of intersecting edges; as opposed to simple; reflex polygon.
concave set  a set of points in which not all segments connecting points of the set lie entirely in the set; nonconvex.
concentric  having the same center, as concentric circles; having the same axis, as concentric cylinders
cone  in general, a cone is the locus of (i.e. surface traced by) the surface formed by lines joining every point of the boundary of a fixed planar closed curve (the base) to a common vertex; commonly, a right circular cone; a "solid cone" is a solid (or the space) bounded by the planar closed curve, called the base, and the line segments connecting the base to the vertex. The area of a solid cone is (1/3)Ah, where A is the area of the base, and h is the height.
congruent  equal; exactly the same (size, shape, etc.). Two figures are congruent when one is the image of the other under a reflection or composite of reflections
conic section  the figure formed by the intersection of a plane and a (right circular) cone
consecutive sides  sides of a polygon that share an endpoint
consecutive vertices  endpoints of a single side of a polygon
constant width  of a bounded figure: having the same width in every direction. A Reuleaux triangle is an example of a figure of constant width. Other examples include the shape formed by an equilateral triangle of side s with circles of radius r around each vertex, and arcs of radius s+r centered on the "far" vertices that join these circles. The same construction can be made with an equilateral (but not necessarily equiangular nsided star, with n an odd number. See Cuttheknot Shapes of Constant Width and Cuttheknot Barbier's Theorem for more information.
convex set  a set of points in which all segments connecting points of the set lie entirely in the set; There are three things one can do to see if a figure is convex  look for "dents", extend the segments (they shouldn't enter the figure), and connect any two points within the figure with a segment (if any part of the segment lies outside the figure, it's concave)
corresponding angles  any pair of angles in similar locations with respect to a transversal
coterminal angles  two angles that have the same terminal side; two angles that differ by an integer multiple of 360 degrees.
cyclic quadrilateral  a quadrilateral whose four vertices lie on a circle.

cyclogon  a curve traced by any vertex of a regular polygon that
rolls without sliding on a straight line. The area under one arch of the
cyclogon (pink, in this diagram) is equal to the area of the polygon plus twice
the area of the circle that circumscribes the polygon. If the "vertices"
of the cyclogon were linked by straight lines instead of curved ones, then the
area under one arch of that figure would be three times the area of the regular
polygon.

cycloid  a curve traced by any point on the circumference of a circle
that rolls without sliding on a straight line. The area under one arch of
the cycloid (blue, in the diagram) is exactly three times the area of the
circle. A curtate cycloid is a curve generated in the same way, except
that the point is inside the circle rather than on the circle. A prolate
cycloid is a curve generated in the same way, except that the point is rigidly
fixed outside the circle. All cycloids, including curtate and prolate
cycloids, are trochoids.
cylinder  in general, a cylinder is the locus of (i.e. surface traced by) a straight line moving parallel to a fixed straight line and intersecting a fixed planar closed curve (the base); commonly, a right circular cylinder
diagonal  a line segment whose endpoints are 2 nonconsecutive vertices of a polygon.
diameter of a circle (or sphere)  the line segment whose endpoints are points on a circle (or sphere) that contains the center of the circle as its midpoint; the length of that line segment
disk  the interior of a circle (an open disk); a circle and its interior (a closed disk)
ellipsoid  a squashed or stretched sphere in which each of the three axes can be of different lengths. (Contrast to a spheroid, in which two of the three axes have the same length.) An ellipsoid has the equation x²/a² + y²/b² + z²/c² = 1

epicycloid  a curve generated by the motion of a point on the circumference of a circle that rolls externally, without slipping, on a fixed circle. Equation: x = (a + b) cos(θ) − b cos[(a + b)θ/b] and y = (a + b) sin(θ) − b sin[(a + b)θ/b]. . . . . . . diagram needs rework.
epitrochoid  a curve generated by the motion of a point rigidly
connected with  but not on the circumference of  a circle that rolls
externally, without slipping, on a fixed circle.
equianglular  having angles of the same measure
equidistant  the same distance from something
equilateral  having sides that are equal in length
even node  a node that has an even number of arcs
exterior angle of a polygon  angle formed by one side and the extension of an adjacent side.
exterior angles  angles outside of two lines cut by a transversal.
face  a polygonal region of a surface
frustum  the basal part of a solid cone or pyramid formed by cutting off the top by a plane parallel to the base; the part of a solid intersected between two usually parallel planes
graph  abstract representation of a set of objects (nodes) where some pairs of the objects are connected by links (edges). See also cage, snark, and lattice. Graphs listed in Wikipedia include Bull, Chv�tal, Clebsch, Desargues,D�rer, Folkman, Foster, Franklin, Frucht, Gray, Gr�tzsch, Hall�Janko, Harries, HarriesWong, Heawood, Higman�Sims, Hoffman�Singleton, McGee, M�bius�Kantor, Null, Pappus, Petersen, Rado, Shrikhande, Tietze's, Tutte�Coxeter; Cages: Balaban 10cage, Balaban 11cage; Snarks: Flower, Szekeres; Lattices: Young�Fibonacci
great circle  the circle formed by the intersection of a sphere and a plane that contains its center and that divides the sphere into two hemispheres
hemisphere  half of a sphere
hypocycloid  a curve generated by the motion of a point on the circumference of a circle that rolls internally (on the concave side), without slipping, on a fixed circle.
hypotenuse  the side opposite the right angle in a right triangle
hypotrochoid  a curve generated by the motion of a point rigidly connected with  but not on the circumference of  a circle that rolls internally (on the concave side), without slipping, on a fixed circle.
icosahedron  a 20sided solid; each side is in the shape of a triangle
included angle  the interior angle made by two consecutive sides of a polygon
included side  the side between two consecutive angles in a polygon
initial side  the side that the measurement of an angle starts from
interior angles  angles between two lines cut by a transversal
isosceles trapezoid  a trapezoid that has a pair of equiangular base angles. (classification of quadrilaterals)
isosceles triangle  a triangle having two sides of equal length
kite  a quadrilateral that has two distinct pairs of consecutive equilateral sides

klein bottle  the 3dimensional analog of the Mobius Strip. The figure is impossible in three dimensions, because one part of its surface passes through another part of its surface without touching it. More on Wikipedia and Mathworld.
lattice  a regular geometrical arrangement of points over an area or in space
lattice graph  a graph whose nodes form a lattice
lattice points  points in the coordinate plane having integer coordinates
leg of a right triangle  a side of a right triangle that include the 90 degree angle
lemniscate  a figure that looks like the "infinity" symbol; a lazy 8; an 8 on its side. This is a crosssection of a torus (a toric section) taken on a plane tangent to the inner circle of the torus. In polar coordinates, the equation of a lemniscate is r=sqrt(cos(2θ)). See mathworld.
linear pair of angles  two supplementary adjacent angles whose noncommon sides form a line
locus  the set of points that satisfy a given condition or equation
major arc  an arc whose endpoints form an angle over 180 degrees with the center of the circle; as opposed to minor arc;
major axis (ellipse)  the line segment containing the foci of the ellipse and whose endpoints are on the ellipse.
measure of an angle  the size of an angle, typically measured in degrees
measure of an arc  The measure of the central angle subtended by an arc.
median  the segment connecting the vertex of an angle in a triangle to the midpoint of the side opposite it
midpoint  the point M on line segment AB such that AM = AB.
minor arc  an arc whose endpoints form an angle less than 180 degrees with the center of the circle.
minor axis (ellipse)  the perpendicular bisector of the line segment whose endpoints are the foci of the ellipse.

M�bius Strip (sometimes spelled Moebius or Mobius)  a surface having
only one side and only one edge. It can be formed from a rectangle by
giving it a half twist, then gluing the edges together.
mouhefanggai  the solid formed as intersection of two perpendicular right circular cylinders of equal diameter. This is also known as a Steinmetz Solid or a bicylinder. Cross sections of a mouhefanggai are square if they are taken parallel to the plane of the axes of the two intersecting cylinders.

ngon  a polygon having n sides.
nonagon  a ninesided polygon.
nonconvex set  a set of points in which not all segments connecting points of the set lie entirely in the set; concave
nonEuclidean geometry  geometry in which given a line and a point on the line, there is either no line (elliptic geometry) or infinitely many lines (hyperbolic geometry) parallel to the given line that pass through the given point.
nonincluded side  the side of a triangle that is not included by 2 given angles.
normal  perpendicular. In particular, a line is normal to a plane
oblique prism or cylinder  a nonright prism or cylinder.
obtuse angle  an angle whose measure is greater than 90 but less than 180 degrees.
octagon  an eightsided polygon
opposite rays  two rays having a common endpoint that form a line.
parallelepiped  a prism whose opposite faces are all parallelograms and congruent (in pairs)
parallelogram  a quadrilateral such that both pairs of opposite sides parallel
pencil  the set of lines through a point, in projective geometry.
pentagon  a fivesided polygon
perimeter of a polygon  the sum of the lengths of the sides of the polygon.
perpendicular bisector  the bisector of a segment perpendicular to it.
perpendicular lines (line segments; rays)  2 lines (line segments; rays) that meet at a 90 degree angle
plane  a flat surface of such nature that a straight line joining two of its points lies wholly in the surface; a plane figure (or a planar figure) is a figure that is entirely within a plane
polygon  a union of 3 or more line segments where each segment intersects 2 other segments, one at each endpoint.
polygonal region  the union of a polygon and its interior.
polyhedron  a closed surface formed by polygonal plane faces, connected at the edges; a "solid polyhedron" is a solid (or the space) enclosed by a polyhedron.
pyramid  a polyhedron having for its base a polygon and for faces triangles with a common vertex (the vertex is not in the plane of the base); a solid pyramid is a solid cone having a polygonal base, so its volume is (1/3)Ah.
quadrangle  a foursided polygon; another name for quadrilateral

quadrilateral  a foursided polygon; special quadrilaterals include rhombus, parallelogram, square, rectangle, trapezoid, isosceles trapezoid, kite.
radius  of a circle or sphere, the line segment (or distance) from the center to the circle or sphere; of a regular polygon, the line segment from the center to a vertex, which is the radius of the circumscribed circle.
ratio of similitude  the ratio of a line segment in figure B to the corresponding line segment in figure A; the ratio of the length of an image to the length of the preimage; ratio of proportionality.
ray  a onedimensional figure that consists of one endpoint A, one point B, all of the points on AB, and all points for which B is between them and A
rectangle  a quadrilateral whose angles are all right angles
rectangular solid  the union of a parallelepiped (box) and its interior.
reference angle  the angle of less than 360 degrees that corresponds to an angle of over 360 degrees; In order to get the reference angle, you must subtract 360 degrees from the given angle until there is less than 360 degrees left.
reflex polygon  a polygon for which 2 or more of its sides intersect each other; complex polygon.
regular polygon  a convex polygon whose angles and sides are all congruent.
regular pyramid  a pyramid whose base is a regular polygon.

rhombus  a quadrilateral having four equilateral sides; equivalently, a parallelogram having four equilateral sides.
right  having a measure of 90 degrees: a right angle; perpendicular; having a right angle: a right triangle; a right cylinder is one in which the straight lines that compose it are normal to its base; a right cone (meaningful only when the base has a center) is one in which the straight line connecting the vertex to the center of the base is normal to the base; a right pyramid (meaningful only when the base has a center) is one in which the straight line connecting the center of the base is normal to the base; a right prism is a prism whose faces meet at right angles.
roulette  a curve traced by a point on a radius or an extension of the radius of a given curve that rolls, without slipping, on a curve, another circle, or a straight line. Cf: trochoid, in which the given curve is a circle.
sagitta  the radius minus the apothem
scalene triangle  a triangle having no equilateral sides.
secant to a circle  a line that intersects the circle in two points
section  the intersection of a threedimensional figure with a plane. e.g. a conic section is the intersection of a cone with a plane.
sector  the interior points of a circle bounded by an arc and two radii
segment (of a circle)  the interior points of a circle bounded by a circular arc and a chord
semicircle  an arc whose endpoints are a diameter of the circle.
septagon  a sevensided polygon.
simple polygon  a polygon whose edges don't cross one another; as opposed to complex;
skew lines  noncoplanar lines. Lines that are neither parallel nor intersecting.
slant height  the length of a lateral edge of a cone
snark  is a connected, bridgeless cubic graph with chromatic index equal to 4. In other words, it is a graph in which every vertex has three neighbors, and the edges cannot be colored by three colors without two edges of the same color meeting at a point. Snarks listed in Wikipedia include Petersen graph, Flower snark, Szekeres snark, Tietze's graph, Blanu�a snarks, Descartes' snark, double star snark.
solid  the union of the surface and the region of space enclosed by a 3D figure; examples: conic solid, cylindric solid, rectangular solid
solid geometry  the study of figures in threedimensional space.
sphere  a closed surface in threedimensional space, every point of which is equidistant from a fixed point, called the center (the center is not part of the sphere)
sphericon  a "cone with a twist". It is created by starting with two identical right circular solid cones (of a particular shape) joined at the base, then slicing them along a plane through both vertices, and then rotating one of the two pieces a quarter turn, and gluing them together.
spheroid  an ellipsoid in which two of the three axes are equal. (Contrast to an ellipsoid, in which all three axes may have different lengths.) A spheroid has the equation (x²+y²)/a² + z²/c² = 1. An oblate spheroid has a polar axis that is shorter than the diameter of the equatorial circle, and can be formed by rotating an ellipse about its minor axis. A prolate spheroid has a polar axis that is longer than the diameter of the equatorial circle, and can be formed by rotating an ellipse about its major axis.
square  an equilateral and equiangular quadrilateral
straight angle  an angle whose measure is 180 degrees; an angle whose sides are collinear.
straight line  a collection of points of such a nature that if one picks any three of them, A, B, and C, the distances AB, AC, and BC will be such that the sum of two of them will equal the third.
supplementary angles  two angles whose measures sum to 180 degrees.
surface  the boundary of a 3D figure.
symmetry diagonal  the diagonal of a kite (a kind of quadrilateral) that perpendicularly bisects the other.
symmetry line  the line of reflection in a reflectionsymmetric figure.
terminal side  the side that the measurement of an angle ends at.
tesselation  a covering of a plane with congruent copies of the same region with no holes or overlaps.
tesseract  the fourdimensional analog of the cube; a 4D hypercube.
tetragon  a foursided polygon; another (rather unusual) word for quadrilateral
three dimensional figure  a set of points in space; examples: box, cone, cylinder, parallelpiped, prism, pyramid, regular pyramid, right cone, right cylinder, right prism, sphere.
torus  a surface or solid shaped like a doughnut and formed by revolving a circle about a line in its plane that does not intersect it.
transversal  a line that intersects two other (usually parallel) lines.
trapezoid  a quadrilateral that has at least one pair of parallel sides
triangle  a polygon having three sides.
trochoid  a curve traced by a point on a radius or an extension of the radius of a circle that rolls, without slipping, on a curve, another circle, or a straight line. Equation: x = aθ − b sin θ, y = a − b cos θ; any of a class of curves including the cycloid, epicycloid, epitrochoid, hypocycloid, and hypotrochoid. Cf: roulette, in which the rolling circle is replaced by any curve.
vertex angle  the angle formed by the equilateral sides of an isosceles triangle.
vertex of a polygon  an endpoint of a segment in a polygon.
vertex of an angle  the common endpoint of the two rays.
vertical angles  2 angles that share a common vertex and whose sides form 2 lines.
width  of a bounded shape: the distance between two parallel lines, each touching the boundary of the shape but not its interior. This is called "the width of the shape in the direction of the lines". Note that figures of constant width (such as a Reuleaux triangle) have the same width in every direction.
zero angle  an angle whose measure is 0. In a zero angle, both the initial and terminal sides are the same.
Cuttheknot Shapes of Constant Width has an applet showing how the Reuleaux triangle has constant width. A "Reuleaux drill" can't make a square hole, but it can make a hole that's almost square, carving out almost 99% of the area of the square.
Cuttheknot Barbier's Theorem for more applets and a general method of making shapes of constant width. Barbier's Theorem states that the circumference of any shape of constant width is equal to πD, where D is the width of the shape.
Number Patterns Fun with Curves and Topology for an animated Reuleaux Triangle.
Mathematical Recreations describes the "cone with a twist", i.e. the sphericon.
Model of a half mouhefanggoid shape with a circular base that has square cross sections.
Thinkquest: Geometry Glossary
The webmaster and author of this Math Help site is Graeme McRae.