Branch of mathematics that studies triangles and the relationships between their sides and the angles between these sides
Trigonometry (from 'triangle', and 'measure') is a branch of mathematics concerned with relationships between angles and side lengths of triangles. In particular, the trigonometric functions relate the angles of a right triangle with ratios of its side lengths. The field emerged in the Hellenistic world during the 3rd century BC from applications of geometry to astronomical studies. Wikipedia