Ear clipping
A simple algorithm that triangulates a polygon by repeatedly clipping off "ears" — convex corners whose triangle is empty.
An ear is three consecutive vertices where the middle one is convex and the triangle they form contains no other vertex of the polygon. The algorithm finds an ear, clips it off as a triangle, and repeats until a single triangle remains.
It is O(n²) but easy to implement and handles arbitrary simple polygons, including concave ones and holes (via bridges). The name was coined by Gary Meisters in 1975. It is the default choice for turning 2D outlines — fonts, UI shapes, navigation polygons — into triangles a GPU can draw.