acute angle	An angle measuring less than 90º.
acute triangle	A triangle with three angles that each measure between 0º and 90º.
angle	A figure formed by the joining of two rays with a common endpoint.
area	The amount of space inside a two-dimensional shape, measured in square units.
circumference	The distance around a circle, calculated by the formula C = d.
complementary angles	Two angles whose measurements add up to 90º.
cone	A solid figure with a single circular base and a round, smooth face that diminishes to a single point.
congruent	Having the same size and shape.
corresponding angles	Angles of separate figures that are in the same position within each figure.
corresponding sides	Sides of separate figures that are opposite corresponding angles.
cube	A six-sided polyhedron that has congruent squares as faces.
cylinder	A solid figure with a pair of circular, parallel bases and a round, smooth face between them.
diameter	The length across a circle, passing through the center of the circle. A diameter is equal to the length of two radii.
equilateral triangle	A triangle with 3 equal sides. Equilateral triangles also have three angles that measure the same.
face	The flat surface of a solid figure.
hypotenuse	The side opposite the right angle in any right triangle. The hypotenuse is the longest side of any right triangle.
isosceles trapezoid	A trapezoid with one pair of parallel sides and another pair of opposite sides that are congruent.
isosceles triangle	A triangle with 2 equal sides and 2 equal angles.
leg	In a right triangle, one of the two sides creating a right angle.
line	A line is a one-dimensional figure, which extends without end in two directions.
line segment	A finite section of a line between any two points that lie on the line.
obtuse angle	An angle measuring more than 90º and less than 180º.
obtuse triangle	A triangle with one angle that measures between 90º and 180º.
parallel lines	Two or more lines that lie in the same plane but which never intersect.
parallelogram	A quadrilateral with two pairs of parallel sides.
perimeter	The distance around a two-dimensional shape.
perpendicular lines	Two lines that lie in the same plane and intersect at a 90º angle.
pi	The ratio of a circle’s circumference to its diameter. Pi is denoted by the Greek letter . It is often approximated as 3.14 or.
plane	In geometry, a two-dimensional surface that continues infinitely. Any three individual points that don't lie on the same line will lie on exactly one plane.
point	A zero-dimensional object that defines a specific location on a plane. It is represented by a small dot.
polygon	A closed plane figure with three or more straight sides.
polyhedron	A solid whose faces are polygons.
pyramid	A polyhedron with a polygonal base and a collection of triangular faces that meet at a point.
Pythagoras	A Greek philosopher and mathematician who lived in the 6th Century B.C.
Pythagorean Theorem	The formula that relates the lengths of the sides of any right triangle: , where c is the hypotenuse, and a and b are the legs of the right triangle.
quadrilateral	A four-sided polygon.
radius	The distance from the center of a circle to any point on the circle.
ray	A half-line that begins at one point and goes on forever in one direction.
rectangle	A quadrilateral with two pairs of parallel sides and four right angles.
rectangular prism	A polyhedron that has three pairs of congruent, rectangular, parallel faces.
rhombus	A quadrilateral with four congruent sides.
right angle	An angle measuring exactly 90º.
right triangle	A triangle containing a right angle.
scalene triangle	A triangle in which all three sides are a different length.
similar	Having the same shape but not necessarily the same size.
sphere	A solid, round figure where every point on the surface is the same distance from the center.
square	A quadrilateral whose sides are all congruent and which has four right angles.
straight angle	An angle measuring exactly 180º.
supplementary angles	Two angles whose measurements add up to 180º.
trapezoid	A quadrilateral with one pair of parallel sides.
triangle	A polygon with three sides.
vertex	A turning point in a graph. Also the endpoint of the two rays that form an angle.
volume	A measurement of how much it takes to fill up a three-dimensional figure. Volume is measured in cubic units.