A specific type of compressor that looks at specific frequency bands of audio and acts on them independently. For example, a multiband compressor can be set to only compress frequencies below 100 Hz, which would prevent a potential build up of low frequency content in a PA system or broadcast. Or it can be used as a mastering tool to aid in adjusting the overall spectral balance of a recording. Keep in mind this is fundamentally different than using a standard compressor with a sidechain. In that case the compressor is always acting on the whole signal, whereas a multiband compressor only acts on specific frequency ranges. One has to be careful, however, because misuse of a multiband compressor can result in skewed tonality. If you compress the high frequencies of a signal going to cassette tape (because you want to print more overall level to tape) to the point where the material ends up sounding dull you have defeated the purpose.