Circumference: the distance around a circle; equals 2 x p x radius or p x diameter (C = 2 p r or p d.).

**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.

D:

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.)

E:

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.

H:

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.

I:

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, v2.

**Isosceles triangle:** a triangle having two equal sides (and thus two equal angles across from those sides).

L:

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

M:

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.

N:

Nonagon: a plane closed figure with nine sides and nine angles

O:

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

P:

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 ( p ):** 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 (a 2 + b 2 = c 2).

Q:

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

R:

Radii: plural of radius.

**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.

S:

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.

T:

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 non parallel 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°.

V:

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.

**Glossary ****Geometry**

**A:**

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.

B:

Bisects: divides into two equal parts.

**C:**

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.