Rapping (or emceeing, MCing, spitting bars, or rhyming) is "spoken or chanted rhyming lyrics". The components of rapping include "content", "flow" (rhythm and rhyme), and "delivery". Rapping is distinct from spoken-word poetry in that it is performed in time to a beat.
Rapping is often associated with and a primary ingredient of hip-hop music, but the origins of the phenomenon can be said to predate hip-hop culture by centuries. Rapping can be delivered over a beat or without accompaniment. Stylistically, rap occupies a gray area between speech, prose, poetry, and singing.
The word (meaning originally "to hit") as used to describe quick speech or repartee predates the musical form. The word had been used in British English since the 16th century, and specifically meaning "to say" since the 18th. It was part of the African-American dialect of English in the 1960's meaning "to converse", and very soon after that in its present usage as a term denoting the musical style.
Today, the terms "rap" and "rapping" are so closely associated with hip-hop music that many use the terms interchangeably.