Geometry Intro Vocabulary

Undefined Terms

The 3 most basic figures in geometry which cannot be defined using other figures: point, line, and plane

Point

Names a location and has no size. It is represented by a dot.

Line

Straight path that has no thickness and extends forever.

Plane

Flat surface that has no thickness and extends forever

Line segment

Part of a line consisting of two points and all points between them

Endpoint

Point at one end of a line segment or the starting point of a ray

Ray

Part of a line that starts at an endpoint and extends forever in one direction.

Opposite rays

Two rays that have a common endpoint and form a line.

Collinear points

Points that lie on the same line

Coplanar points

Points that lie in the same plane

Postulate

A statement that is accepted as true without proof

Congruent segments

Segments that have the same length

Midpoint

The point on a line segment that bisects, or divides, the segment into two congruent segments.

Angle

Figure formed by two rays, or sides, with a common endpoint.

Vertex

The common endpoint of an angle. (A point where two lines meet)

Measure of an angle

Given in degrees using protractor with a range from 0 degrees to 180 degrees.

Acute Angle

An angle that measures greater than 0 degrees and less than 90 degrees

Right Angle

An angle that measures 90 degrees

Obtuse Angle

An angle that measures greater than 90 degrees and less than 180 degrees

Straight Angle

An angle formed by two opposite rays and measures 180 degrees

Congruent Angles

Angles that have the same degree measure

Angle Bisector

A ray that divides an angle into two congruent angles

Complementary Angles

Two angles whose measures add up to 90 degrees

Supplementary Angles

Two angles whose measures add up to 180 degrees

Adjacent Angles

Two angles in the same plane with a common vertex and a common side but no common interior points

Linear Pair Angles

A pair of adjacent angles whose noncommon sides are opposite rays forming a line

Vertical Angles

Two nonadjacent angles formed by two intersecting lines

Parallel Lines

Coplanar lines that do not intersect

Perpendicular Lines

Coplanar lines that form right angles-90 degree angles.

Degree

A measure of an angle based upon 360 degrees in a circle. 1 degree = 1/360 of a circle