I don't normally site urban dictionary but why not?
A b-side was originally known as the song on the flipside of a 12" or 7" vinyl single that backed the A-side, but wasn't the song promoted or received the radio play. Today CD singles have b-sides along with remixes, even though ironically CD singles only have one side that is used. With CD singles dying a slow death today, b-sides are becoming a rarer commodity.