Collision detection
Also said as Broad phase, Narrow phase, Spatial partitioning, AABB
Working out which things are touching. Done in two passes: a broad phase that quickly throws out everything that is nowhere near, and a narrow phase that does the exact test on what is left.
Testing every object against every other grows with the square of the count: 100 objects need 4,950 tests, and 1,000 need nearly 500,000. Without a broad phase, collisions alone will eat the frame.
Say it
“We have 800 enemies and are testing every pair. Put them in a uniform grid with 64 px cells and only test neighbouring cells.”
Where you’ve seen it
Doom cuts each level into a blockmap, a grid of 128 by 128 unit cells, so a moving thing only checks what is listed in the cells around it.
Not to be confused with collision response
Detection answers whether two things are touching. Response decides what happens next: bounce, stop, take damage.