Multiband compression
Splitting the signal into frequency bands and compressing each one separately, so a booming bass line doesn't drag the treble down with it.
An ordinary compressor turns the whole signal down when anything crosses the threshold, and since bass carries the most energy, bass usually ends up in charge. The multiband version first splits the signal with crossover filters, typically into three to five bands, gives each its own threshold and ratio, and sums them back together. A de-esser is effectively a single one of those bands.
Radio got there first: Orban's Optimod-FM of 1975 made multiband processing standard on FM, and is why stations sounded equally loud whatever record was on. In mastering the tool is as powerful as it is easy to abuse — set wrong, it changes tonal balance dynamically, which is far harder to hear than a static EQ move.