Subdivision surface
A smooth surface produced by repeatedly splitting and smoothing a coarse control mesh.
Rather than modelling every polygon, you build a sparse control cage and let a rule refine it: each step splits the faces, adds new points, and nudges the old ones toward a weighted average of their neighbours. After a few iterations the result approaches a mathematically smooth limit surface.
Artists get soft shapes without hand-managing millions of vertices, and the level can be chosen at render time. Catmull–Clark dominates for quad meshes, Loop for triangles. The technique took off in film (Geri's Game, 1997) and only became game-ready once GPU tessellation could do the refinement in real time.