Usually used to describe a song without vocals. Also known as Backing Music
The part of the song which is just the intstruments is called the "Backing Track" or "Instumentals".
It is useful to get these tracks if you wish to spoof a song, but make it have the same beats/sounds in the background as the original.
That singer is really bad, but I like the tune. I think I will just try and get the backing track and record my own voice over it. That'd rock!