Convolution reverb
Reverb made by convolving a sound with a recording of how a real space responds to a short, sharp bang.
A room's acoustics can be captured as an impulse response: record what happens when it is hit with a starter pistol, a bursting balloon or, more precisely, a sine sweep that is deconvolved afterwards. Convolve any dry sound with that response and it seems to have been played in the room — a cathedral, a car interior, the inside of an oil drum.
The maths is simple but heavy: every sample is multiplied against the entire impulse response, which for a three-second hall gets absurdly expensive. The fix is block-based FFT convolution. Sony shipped the first hardware unit, the DRE-S777, in 1999, and Audio Ease's Altiverb turned the technique into a plugin a few years later.