Dictionary definition: "to sing without musical accompaniment". Like Janis Joplin singing "Mercedes-Benz", or you in the shower (and would you mind either keeping it down or learning the concept of staying in key? Thank you. And no more Counting Crows! OK? Sheesh!). But it's also used to describe doo-wop, which bends that definition a little: the group becomes the accompaniment for a soloist (usually its lead singer), in lieu of a backing band.