**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 (a^{2} + b^{2} = c^{2}).

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

**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.

**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.