"A cappella" singing is the oldest form of music. The term refers to unaccompanied (ie. without instruments) choral or individual singing. Literally it is an Italian phrase meaning 'in the manner of the church' - a byproduct of its liturgical development. However there are indications of a cappella singing in the archaeological record from over three thousand years ago and it is certainly likely that some form of human vocal performance predates all but the simplist instruments.
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. As a result Sing a Cappella! has chosen to focus specifically on "contemporary a cappella" by which we mean songs written in the 20th Century and beyond. This is not because we consider earlier a cappella works inferior - however there are some good reasons for focussing on this time period:
the majority of contemporary a cappella singing is not of original works but of songs familiar to both singers and audiences. This encourages a popular and "built in" audience for the art form (although can lead to complications over copyright)
developments in harmonic and rythmic form during this period have led to some interesting music which is great fun to sing (but that's not to decry in any way earlier polyphonic music)
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 rythmic 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). Such fusions of singing and technology are considered valid a cappella provided the basic input to the process remains solely the human voice.
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 - groups to whom those of us who sing in UK contemporary a cappella choirs owe a great debt. Their influence has been supplemented by the rise of a competitive student a cappella movement from the USA - particularly supported by prolific arranger and educator Deke Sharon who many consider the Godfather of the art form. In the UK competitive a cappella singing is managed through The Voice Festival UK - a charitable trust founded specifically to promote a cappella singing.