acute angle an angle whose measure is less than 90°.

acute triangle a triangle containing all acute angles.

adjacent angles: angles that share a common side and a common vertex.

angle formed by two rays with a common endpoint.

arc the set of points on a circle that lie in the interior of a central angle.

area the space within a shape; measured in square units.

bisects divides into two equal parts.

central angle an angle whose vertex is the center of the circle. The measure of a central angle is equal to the measure of its arc.

chord a line segment joining any two points on a circle.

circle in a plane, the set of points all equidistant from a given point.

circumference the distance around a circle; equals 2 x π x radius or π x diameter (C = 2πr or πd.).

complementary angles two angles the sum of whose measures is 90°.

concave polygon a polygon which contains at least one diagonal outside the figure.

concentric circles circles with the same center.

congruent exactly alike. Identical in shape and size.

consecutive next to each other

convex polygon a polygon in which all diagonals lie within the figure.

corresponding in the same position. Coinciding.

cube a six-sided solid. All sides are equal squares and all edges are equal.

decagon a plane closed figure with ten sides and ten angles.

degree a unit of measurement of an angle.

diagonal of a polygon a line segment connecting one vertex to another vertex, and not a side of the polygon.

diameter a line segment that contains the center and has its endpoints on the circle. Also, the length of this segment. (A chord through the center of the circle.)

equilateral triangle a triangle in which all three angles are equal in measure and all three sides have the same length.

exterior angle an angle formed outside the polygon by extending one side. In a triangle, the measure of an exterior angle equals the sum of the measures of the two remote interior angles.

height altitude. From the highest point, a perpendicular drawn to the base.

heptagon a plane closed figure with seven sides and seven angles.

hexagon a plane closed figure with six sides and six angles.

hypotenuse in a right triangle, the side opposite the 90° angle.

inscribed angle in a circle, an angle formed by two chords. Its vertex is on the circle. The measure of an inscribed angle equals one-half the measure of its arc.

interior angles angles formed inside the shape or within two parallel lines.

intersecting lines lines that meet at a point.

isosceles right triangle a triangle having two equal sides, two equal angles, and one 90° angle. Its sides are always in the ratio 1, 1, √2.

isosceles triangle a triangle having two equal sides (and thus two equal angles across from those sides).

legs in a right triangle, the two sides forming the 90° angle. In a trapezoid, the nonparallel sides.

line segment a part of a line; has two endpoints

median in a triangle, a line segment drawn from a vertex to the midpoint of the opposite side.

median in a trapezoid, a line segment parallel to the bases and bisecting the legs.

midpoint the halfway point of a line segment, equidistant from each endpoint.

minute a subdivision of an angle, one-sixtieth of a degree.

nonagon a plane closed figure with nine sides and nine angles

obtuse angle an angle greater than 90° but less than 180°

obtuse triangle a triangle containing an obtuse angle

octagon a plane closed figure with eight sides and eight angles

parallel lines two or more lines, always the same distance apart. Parallel lines never meet.

parallelogram a four-sided plane closed figure having opposite sides equal and parallel. (Opposite angles are equal, and consecutive angles are supplementary.)

pentagon a five-sided plane closed figure. The sum of its five angles is 540°.

perimeter the total distance around the outside of any polygon. The total length of all the sides.

perpendicular lines two lines that intersect at right angles.

pi (π) a constant used in determining a circle's area or circumference. Equals approximately 3.14 or 22/7

plane often described as a flat surface.

plane figure shape having only length and width (two dimensional).

plane geometry the study of shapes and figures in two dimensions.

point a basic element of geometry, a location. If two lines intersect, they do so at a point.

polygon many-sided plane closed figure. Triangle, quadrilateral, pentagon, and so on.

prism a three-dimensional shape bounded by congruent parallel faces and a set of parallelograms formed by joining the corresponding vertices of the bases.

Pythagorean theorem a theorem that applies to right triangles. The sum of the squares of a right triangle's two legs equals the square of the hypotenuse (a2 + b2 = c2).

quadrilateral a four-sided plane closed figure. The sum of its four angles equals 360°.

radius a line segment whose endpoints lie one at the center of a circle and one on the circle. Also, the length of this segment.

ray a half-line. Continues forever in one direction. Has one endpoint.

rectangle a four-sided plane closed figure having opposite sides equal and parallel and four right angles.

regular polygon a polygon in which sides and angles are all equal. For example, a regular pentagon has five equal angles and five equal sides.

rhombus a parallelogram with four equal sides.

right angle an angle whose measure is equal to 90°.

right circular cylinder a solid shaped like a can. Base meets side at a right angle.

right triangle a triangle containing a 90° angle.

scalene triangle a triangle having none of its sides equal (or angles equal).

similar having the same shape but not the same size, in proportion.

solid geometry the study of shapes and figures in three dimensions: length, width, and thickness.

square a four-sided plane closed figure having equal sides and four right angles. Its opposite sides are parallel.

straight angle an angle equal to 180°. Often called a line.

straight line often described as the shortest distance between two points. Continues forever in both directions. (Line means straight line.)

supplementary angles two angles the sum of which measures 180°.

surface area the total surface of all sides of a solid, or the total area of faces.

tangent to a circle a line, line segment, or ray that touches a circle at one point (cannot go within the circle).

transversal a line crossing two or more parallel or nonparallel lines in a plane.

trapezoid a four-sided plane closed figure with only one pair of parallel sides, called bases.

triangle a three-sided plane closed figure. Contains three angles the sum of whose measures is 180°.

vertex the point at which two rays meet and form an angle, or the point at which two sides meet in a polygon.

vertical angles the opposite angles formed by the intersection of two lines. Vertical angles are equal in measure.

vertices plural of vertex.

volume capacity to hold, measured in cubic units. Volume of rectangular prism = length x width x height.