An mp3 release that is compiled from different rips or sources. This was common during the Napster era of file sharing, where you had to download songs individually.
For example, you download tracks 1-5 from one site, then got disconnected, so you got tracks 6-14 somewhere else. You end up with an album that most likely has inconsistent bitrates and encoding techniques, and could even be from different releases of the album (remasters, etc).
I had an OCD attack when I found out that the rarest mp3s in my collection were mutt rips.