Compand is a contraction of compress/expand. Used in a couple of contexts:
1. When transmitting radio (UHF or FM) signals, you can compress the dynamic range of your signal, and expand it when they arrive at the receiver.
2. When messing around with your music files, you may want to change their dynamic range -- for example, in order to amplify the quietest sounds in the file while keeping them in proper proportion to the rest of the music (i.e. flatten or compress the dynamics -- this will result in lower fidelity, but may be useful if you want to listen to the music in your car or other loud environment in which the quieter sounds would not be audible). Some audio file manipulation programs (e.g. sox) will have a Compand option, which can be used to flatten dynamics in this way.
Compand, Companding, Compander.