Geometry Definitions

Straight Angle

Exactly 180 degrees

Line

Infinite set of points; extends infinitely in two directions

Congruent

Same size and shape

Bisector of a Segment

A line or ray that passes through the midpoint of a segment

Relationship Between Two Points

Always collinear; Exactly one line that contains them both; Always coplanar; Infinite planes that connect them both

Congruent Angles

Angles with the same measure

Acute Angle

Between 0 and 90 degrees

Obtuse Angle

Between 90 and 180 degrees

Adjacent Angles

Coplanar angles that share a common vertex and a common side, but have no interior points in common.

Bisector of an Angle

Divides it into two congruent angles

Right Angle

Exactly 90 degrees

Plane

Flat surface extending infinitely in all directions

Point

Location; no size or shape; 0-dimensional

Postulate

Mathematical statement accepted without proof

Line Segment

Part of a line; two endpoints and all the points inbetween

Ray

Piece of a line with a beginning and no end; one endpoint; extends infinitely in one directions

Non-Collinear Points

Points not on the same line

Non-Coplanar Points

Points not on the same plane

Collinear Points

Points on the same line

Coplanar Points

Points on the same plane

Congruent Segments

Segments with the same length

Relationship Between Four Points

Sometimes collinear; Sometimes coplanar; Sometimes coplanar but not collinear; sometimes non-coplanar; Four non-coplanar points determine space

Relationship Between Three Points

Sometimes collinear; Sometimes coplanar; Three non-collinear points determine a plane

Midpoint of a Segment

The point that divides a segment into two congruent segments

Space

The set of all points

Angle

The union of two rays with a common endpoint (vertex)