Theta

Theta ($\theta$) is a symbol used to denote the unknown measure of an angle.

It is used mostly in displaying the trigonometric ratios of sine, cosine, and tangent. For example: 

$\sin \theta ={\frac {\mbox{opp}}{\mbox{hyp}}}$

$\cos \theta ={\frac {\mbox{adj}}{\mbox{hyp}}}$

$\tan \theta ={\frac {\mbox{opp}}{\mbox{adj}}}$