A girl group is a music act featuring three or more female singers who generally harmonize together. The term "girl group" is also used in a narrower sense in the United States to denote the wave of American female pop music singing groups that flourished in the late 1950s and early 1960s between the decline of early rock and roll and start of the British Invasion, many of whom were influenced by doo-wop. All-female bands, in which members also play instruments, are usually considered a separate phenomenon. These groups are sometimes called "girl bands" to differentiate, although this terminology is not universally followed.