(noun) The current trend in which music is produced and mastered to be as loud as possible so that it "stands out" on radio and other media. This is generally achieved by compressing the audio waveform so that the overall loudness of the wave can be increased. Compression literally squeezes the dynamic range (and therefore the impact) out of a piece of music.
Californication by the Red Hot Chili Peppers is a great example of an excellent album compromised by poor mastering. The loudness war is killing music.