Definition of a cappella in US English:

a cappella

adjective & adverb

  • (with reference to choral music) without instrumental accompaniment.

    as adjective ‘an a cappella Mass’
    as adverb ‘the trio usually performs a cappella’
    • ‘We'll prepare a few songs that we can do a cappella.’

Origin

Italian, literally ‘in chapel style’.

Pronunciation