Convex hull
The smallest convex shape enclosing a set of points — think of a rubber band stretched around nails in a board.
Classic algorithms are Graham scan (1972), gift wrapping (Jarvis march, 1973), QuickHull and Chan's algorithm, which reaches the optimal O(n log h). In 2D the whole hull is a polygon; in 3D a polyhedron.
In games and physics, collision shapes are often built as the convex hull of a model, because convex-versus-convex testing via GJK is cheap and robust. Concave models are first broken apart with convex decomposition.