Alternate exterior angles
Exterior angles on alternate sides of the transversal (not on the same parallel line)

Alternate interior angles
Interior angles on alternate sides of the transversal (not on the same parallel line)

Complementary angles
Two positive angles that when added give 90 degrees.

Corresponding angles
Any pair of angles in similar locations with respect to a transversal.

Linear pair
2 supplementary adjacent angles whose non-common sides form a line.
 

Parallel lines
Two or more coplanar lines that never intersect.

Perpendicular lines
Two segments, rays, or lines that form a 90 degree angle.

Postulate
A statement assumed to be true without proof.

Proof
A sequence of justified conclusions used to prove the validity of an if-then statement.

Supplementary angles
Two positive angles whose measures add to 180 degrees.

Theorem
A theorem in mathematics is a proven fact. A theorem about polygon must be true for every polygon; there can be no exceptions. An idea which works in  several different cases is not enough.

Transversal
A line that intersects  others.