"A cappella" singing is the oldest form of music. The term refers to unaccompanied (ie. without instruments) choral or individual singing. In Italian phrase meaning 'in the manner of the church'.
Any music can be considered an "a cappella" arrangement provided it does not contain instrumental notation, and the sheer diversity of what can be encompassed by the term can be quite dizzying!
The majority of contemporary a cappella singing is not of original works but of songs familiar to both singers and audiences. In recent years there have been a number of developments in contemporary a cappella including the rise of "beatboxing" (percussive sounds created by trained practitioners and used to enhance the rhythmic elements of an arrangement) and various post-production technologies which can now be applied to live performance (such as mixing, multi-tracking, looping, effects and pitch modulation).
In the UK mainstream presentation of a cappella in the media has been spearheaded by a number of pioneering vocal groups such as The Kings Singers, The Flying Pickets, The Swingle Singers and The Magnets. The recent “Pitch Perfect” films and various TV talent shows have brought a cappella singing to mass audiences and it now has true mainstream appeal!