- acute angle: an angle whose measure is greater than 0 degrees and less than 90 degrees
- angle: a geometric figure formed by two rays that have a common endpoint
- bisect: to cut into two congruent parts
- collinear: points that lie on the same line
- complementary angles: two angles whose sum measures 90 degrees
- congruent: having the same size and shape
- coplanar: any three or more points that lie in the same plane
- intersect: to meet at a point
- line: an infinite number of points that goes on forever in both directions
- line segment: an infinite number of points that has two distinct end points
- midpoint: The point that divides a segment into two congruent segments.
- obtuse angle: an angle whose measure is greater than 90 degrees and less than 180
- perpendicular: lines that intersect to form right angles
- plane: a flat surface of points that goes on forever in all directions
- point: a location in space (it has no dimensions)
- ray: an infinite set of points that starts at a point and goes on forever
- right angle: an angle formed by two perpendicular lines, the measure of which is 90 degrees
- straight angle: an angle whose measure is 180 degrees
- supplementary: two angles whose sum measures 180 degrees
- vertex: the point of intersection of the two rays that form the sides of the angle