Limiter (audio)

A compressor turns down whatever crosses the threshold by some proportion; a limiter simply doesn't let it cross. The digital "brickwall" kind reads a few milliseconds ahead, so it can pull the level down before the peak arrives rather than reacting after the damage is done. The delay this adds is absorbed by the DAW's plugin delay compensation.

The Waves L1 of 1994 made the limiter mastering's default tool: squash the peaks, raise everything else, and your record sounds louder than the competitor's. That was the opening shot of the loudness war. Modern limiters oversample internally to catch peaks that form between samples — otherwise the ceiling in dBFS is little more than a hopeful estimate.