This question is fraught with conflation. Everyone here seems to be trying to shoehorn answers to fit with the original question. When the term came about, 'single' was not intended to connote that you would only be buying one song. Music was first distributed on vinyl records. You could purchase vinyl in three options:
- LP, or Long-Playing. This vinyl record consisted of roughly 5 songs per side. It was spun on a record player at 33 1/3 revolutions per minute (RPM)
- EP, or Extended-Play. This vinyl consisted of a number of total songs less than an LP, but more than a single.
- Single. The intent on releasing a single was putting the best foot forward to promote an entire album, or LP. Singles were only a song in length on each side. Because there was less content, it was recorded differently (to take up more vinyl real estate), and ran at 45 RPM. Some LPs had strength enough to release multiple singles. (Michael Jackson had a number of LPs where this was true)
The single, or reason that this single song was released, was consider the A side. The B side almost always contained a weaker (and different) song, used to draw interest on the rest of the album. Over time, artists would put songs on the B side that would never be contained on a studio LP, to generate rarity and more reason to purchase singles.