Compressor (audio)
An effect that turns the volume down automatically when the signal gets too loud, shrinking the gap between quiet and loud.
Nothing to do with zip files — here it is dynamics being squeezed. Four controls run the show: threshold (where it starts acting), ratio (how hard), and attack and release (how fast it grabs and lets go). Makeup gain then lifts everything back up, and the result sounds denser and louder. Push the ratio to infinity and you have a limiter, which lets nothing past the ceiling.
The classics are analogue: the Teletronix LA-2A with its light-sensitive cell and the Urei 1176 from 1967, both cloned in countless plugins. Overuse through the 1990s and 2000s produced the loudness war, with records squashed until the transients disappeared. Measuring in LUFS and streaming-service normalisation have since made that war pointless.