Introductory Geometric Vocabulary
Angles
Lines
Circles
Parts of a line 
Pairs of lines 
Parallels cut by a transversal 
Line
Line segment: a bounded, connected subset of a line.
Ray: connected part of a line extending infinitely in one direction.
By measure 
Parts of a circle 
The radius of a circle C is a line segment with two endpoints: 1) the center and 2) some point on C.
Circumference: total length of a circle (C=2πr)
The center of a circle is the point which lies an equal distance from every point on the circle.
Corresponding angles
Pairs of angles 
Parallel lines are lines in the same plane that do not intersect (no matter how far extended).
Intersecting lines have one (and only one) point in common.
Perpendicular lines intersect at right angles.
Right angles measure 90°.
Acute angles measure <90°.
Obtuse angles measure >90° and <180°.
Straight angles measure 180°.
Complementary angles add up to 90°.
Supplementary angles add up to 180°.
Vertical angles: angles opposite each other where two lines intersect.
Transversal: a line that intersects two parallel lines.
The diameter of C is the longest possible line segment between two points on C. Or, the length of that segment. (d=2r)