Trigonometry as a mathematical discipline metamorphosed over time.  Watch it grow!

It began as a study of right-angled triangles, largely for surveying and as a fun exercise.

In time, we learned how to study triangles of any shape, and this was a great aid to navigation, for example.

As more questions were asked, the definitions changed and now trigonometry is used in the analysis of any regular (cyclic) motion.  It is also used extensively in abstract algebraic analysis.