Broad phase
The first stage of a physics engine's collision search, cheaply discarding object pairs that cannot possibly touch.
With n objects in a scene there are roughly n²/2 possible pairs, and testing each one exactly becomes hopeless at a few hundred objects. The broad phase instead compares approximate bounding volumes — usually axis-aligned boxes — and hands back a short list of candidate pairs that actually overlap. The exact test is left to the narrow phase.
Classic methods include sweep and prune (sort each object's interval along an axis and sweep, described by Cohen et al. in I-COLLIDE, 1995), uniform grids, spatial hashing and bounding volume hierarchies. The right choice depends on how objects are spread: a thin, sprawling field suits sweep and prune, a dense cluster suits a grid.