Upward compression
Dynamic range compression that lifts the quiet parts below the threshold rather than pushing the loud parts down.
A conventional compressor works downwards: anything over the threshold gets turned down. Upward compression does the opposite, raising everything below the threshold while leaving the transients alone. Dynamic range shrinks just the same, but the crack of a snare survives instead of being flattened.
Few classic hardware units did it in pure form, so engineers approximated the effect with parallel compression. In software it went mainstream with Xfer's free OTT plugin, a multiband compressor that pushes both ways and became, in electronic music, shorthand for "everything audible, all the time". The downside is predictable: what gets lifted includes hiss, room tone and breaths, so the noise floor rides up along with it.