A remix is often the vocal part of a song mixed with the instrumental part of another song, or more than one song.
The 2 major types of vocal tracks are:
a cappella [a kkapˈpɛlla] or acapella = a vocal song or section intended to be heard solo (in latin "a capella" means "in the chapel", so it is the song as one could hear in a chapel, with no instrumental backing),
isolated vocal track = a vocal track originally not intended to be heard solo. It may be a separate recording of a microphone (or several microphones) in studio/performance or the result of a processing to extract the voice from an audio track merging the voice(s) and musical instruments,
For the sake of clarity, vocal tracks are hereafter called acapellas even when they are isolated vocal tracks.