Limiter

A limiter is a compressor with an extreme ratio and a very fast attack, aimed at one job: nothing gets past this line. Set a ceiling, and peaks that would have crossed it are pulled down instead. A brickwall limiter holds that line in whatever domain it is measuring, usually by looking ahead a few milliseconds so it can start ducking before the transient arrives rather than reacting late. Note the qualifier: a limiter watching sample peaks guarantees only the samples. The reconstructed waveform between them, and the file after a lossy encode, can still poke above the ceiling.

Two ways people use it. As a safety limiter, parked last in the chain with a ceiling at -1 dBTP, barely touching anything, just catching the one unexpected shout. As a maximizer, driving the input up into the limiter so the whole thing gets louder while the ceiling holds, which is how mastering hits a LUFS target. Both are the same plugin, different amounts of gain reduction.

Gotcha: a limiter is not a fix for clipping that already happened, and leaning on one hard flattens transients until drums lose their snap and speech sounds airless and fatiguing. If you are pulling more than 3 to 4 dB of reduction on the master to reach a target, the mix is the problem, not the ceiling. Also switch the ceiling into true peak mode and then measure the exported, encoded file, because that is the only place the real answer shows up.