Sutherland–Hodgman
The algorithm that clips a polygon against a convex region by shaving off whatever lies outside one edge at a time.
Ivan Sutherland and Gary Hodgman published "Reentrant Polygon Clipping" in 1974. The idea is a pipeline: clip the polygon against one plane, hand the result on, clip it against the next, until every edge of the clip region is done. Each polygon edge falls into one of four cases — both ends inside, both outside, or one of each — which decide which vertices pass through and where new ones are created.
The clip region has to be convex; a concave input can leave degenerate edges along the boundary, which rarely bothers a rasterizer. The technique lives on wherever triangles cross the view frustum's near plane, and in physics engines such as Box2D, which clip the incident face against the side planes of the reference face to build a contact manifold.