Envelope follower
A circuit or algorithm that turns an audio signal into a slow-moving curve of how loud it is right now.
The recipe is rectify, then smooth: take the absolute value of the waveform and low-pass filter it, with separate times for how fast the curve rises and falls. What comes out is no longer audio but a control signal. Peak detection tracks every transient; RMS detection lands closer to perceived loudness.
Every compressor, noise gate and limiter has one in its detector. Wire it to a filter and you get an envelope filter, or auto-wah — the Musitronics Mu-Tron III from 1972 being the guitarist's classic. A vocoder is essentially a bank of envelope followers, one per frequency band, each steering the matching band of the carrier.