undefined term
A basic figure that is not defined in terms of other figures. The undefined terms in geometry are point, line, and plane.
point
An undefined term in geometry, it names a location and has no size.
line
An undefined term in geometry, a line is a straight path that has no thickness and extends forever.
plane
An undefined term in geometry, it is a flat surface that has no thickness and extends forever.
collinear
Points that lie on the same line.
coplanar
Points that lie in the same plane.
segment
A part of a line consisting of two endpoints and all points between them.
endpoint
A point at an end of a segment or the starting point of a ray.
ray
A part of a line that starts at an endpoint and extends forever.
opposite rays
Two rays that have a common endpoint and share a line.
postulate
A statement that is accepted as true without proof. Also called an axiom.
coordinate
A number used to identify the location of a point. On a number line, one coordinate is used. On a coordinate plane, two coordinates are used, called the x-coordinate and the y-coordinate. In space, three coordinates are used, called the x-coordinate, the
distance between two points
The absolute value of the difference of the coordinates of the points.
length
The distance between the two endpoints of a segment.
congruent segments
Two segments that have the same length.
construction
A method of creating a figure that is considered to be mathematically precise. Figures may be constructed by using a compass and straightedge, geometry software, or paper folding.
between
Given three points A, B,and C, B is between A and C if and only if all three of the points lie on the same line, and AB + BC = AC.
midpoint
The point that divides a segment into two congruent segments.
bisect
To divide into two congruent parts.
segment bisector
A line, ray, or segment that divides a segment into two congruent segments.
angle
A figure formed by two rays with a common endpoint.
vertex of an angle
The common endpoint of the sides of the angle.
interior of an angle
The set of all points between the sides of an angle.
exterior of an angle
The set of all points outside an angle.
measure of an angle
Angles are measured in degrees. A degree is 1/360 of a complete circle.
degree
A unit of angle measure; one degree is 1/360 of a circle.
acute angle
An angle that measures greater than 0� and less than 90�.
right angle
An angle that measures 90�.
obtuse angle
An angle that measures greater than 90� and less than 180�.
straight angle
A 180� angle.
congruent angles
Angles that have the same measure.
angle bisector
A ray that divides an angle into two congruent angles.
adjacent angles
Two angles in the same plane with a common vertex and a common side, but no common interior points.
linear pair
A pair of adjacent angles whose noncommon sides are opposite rays.
complementary angles
Two angles who have a measure of 90�.
supplementary angles
Two angles whose measures have a sum of 180�.
vertical angles
The nonadjacent angles formed by two intersecting lines.
perimeter
The sum of the side lengths of a closed plane figure.
area
The number of nonoverlapping unit squares of a given size that will exactly cover the interior of a plane figure.
base of a triangle
Any side of a triangle.
height of a figure
The length of an altitude of the figure.
diameter
A segment that has endpoints on the circle and that passes through the center and that passes through the center of the circle; also the length of that segment.
radius of a circle
A segment whose endpoints are the center of a circle and a point on the circle; the distance from the center of a circle to any point on the circle.
circumference
The distance around a circle.
pi
The ratio of the circumference of a circle to its diameter, denoted by the Greek letter ? (pi). The value of ? is irrational, often approximated by 3.14 or 22/7.
coordinate plane
A plane that is divided into four regions by a horizontal line called the x-axis and a vertical line called the y-axis.
leg of a right triangle
One of the two sides of the right triangle that form the right angle.
hypotenuse
The side opposite the right angle in a right triangle.
transformation
A change in the position, size, or shape of a figure or graph.
preimage
The original image in a transformation.
image
A shape that results from a transformation of a figure known as the preimage.
reflection
A transformation across a line, called the line of reflection, such that the line of reflection is the perpendicular bisector of each segment joining each point and its image.
rotation
A transformation about a point P, also known as the center of rotation, such that each point and its image are the same distance from P. All of the angles with vertex P formed by a point and its image are congruent.
translation
A transformation that shifts or slides every point of a figure or graph the same distance in the same direction.