a 7" record released by two bands, where each band contributes one song. common among small indie/emo/punk bands and labels. A really good way to find new bands.
Occasionally there will be more bands or songs - for example the Mineral / Sensefield / Jimmy Eat World 7" on Crank! Records. Sometimes, more commonly, each band will cover a song by the other band - for example, the Built to Spill / Marine Research 7", which has Built to Spill playing Marine Research's "By the Way", and Marine Research playing Built to Spill's "Sick and Wrong".