Definition: A compressor is an audio tool, either software or hardware, that allows a signal's sound level to be reduced by a certain user-defined level when the input level reaches a user-defined threshold. Common uses are leveling vocals, making drums sound "tighter" and "fatter", and helping complete mixes be more balanced and smooth.
Also Known As: signal leveling, limiting
Examples:
I put a compressor on the bass guitar channel to help even it out in the mix.