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Geometry Guide
Basic Terms and Definitions
Term
Point
Line
Plane
Space
Collinear
Coplanar
Segment
Ray
Opposite rays
Intersection
Congruent
Congruent segments
Segment midpoint
Segment bisector
Angle
Acute angle
Obtuse angle
Right angle
Straight angle
Congruent angles
Angle bisector
Adjacent angles
Vertical angles
Definition
A location or position. A point has no dimension. A point has no
length, width, or height.
An infinite set of points that extend in two opposite directions. A line
has length (infinite), but no width, or thickness.
An infinite set of points that form a flat surface extending in all
directions. A plane has length and width (infinite), but no thickness.
The set of all points.
Collinear points lie on the same line. Noncollinear points do not lie on
the same line.
Coplanar points lie on the same plane. Noncoplanar points do not lie
on the same plane.
A section of a line designated by two endpoints and the set of all points
between them.
A section of a line with one endpoint and extending in one direction.
Two rays with the same endpoint that form a straight line.
The set of points in both objects.
Two objects are congruent if they have the same size and shape.
Two segments are congruent if they have the same length.
A midpoint divides a segment into two ≅ segments.
A line, ray, or segment that intersects a segment at its midpoint.
A figure formed by two rays with the same endpoint. The endpoint is
the vertex. The rays form the sides of the angle.
∠A is acute if 0 < m∠A < 90
∠A is obtuse if 90 < m∠A < 180
∠A is a right angle if m∠A = 90
∠A is a straight angle if m∠A = 180
Two angles are congruent if they have equal measure.
A ray that divides an angle into two congruent angles.
Two angles are adjacent if they have:
1. Common vertex
2. Common side
3. No points in common
The two angles “across” from each other at the intersection of lines.
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