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°.
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.
