noun The branch of mathematics that deals with the relationships between the sides and angles of (in particular) right-angled triangles, as represented by the trigonometric functions, and with calculations based on said relationships.
Trigonometry emerged in the Hellenistic world during the 3rd century BCE from applications of geometry to astronomy; the Greeks focused on the calculation of chords, while mathematicians in India created the earliest known tables of values for trigonometric functions such as sine.