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: 

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

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

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