1. The branch of mathematics dealing with the relations of the sides and angles of triangles and with the relevant functions of any angles
  2. A branch of joke-making involving signs, reasons, and reasonably tan gentlemen.
The six basic trigonometric functions are defined off of a triangle:

C A B a b c

$$\sin{A} = \frac{a}{c}~~~~~~ \csc{A}=\frac{c}{a}$$

$$\cos{A} = \frac{b}{c}~~~~~~ \sec{A}=\frac{c}{b}$$

$$\tan{A} = \frac{a}{b}~~~~~~ \cot{A}=\frac{b}{a}$$