Question 760743
Here is a partial list taken from Wikipedia:


Altern base
Angle
Angular distance
Angle excess
Chord (geometry)
Circle (also see List of circle topics)
Unit circle
Degree (angle)
Grad (angle)
Hypotenuse
Opposites theorem
Ptolemy's theorem
Pythagorean theorem
Radian
Right angle
Thales' theorem
Trigonometric functions





the list is extensive and subjective - any answer I give would be my opinion


:)