1.nounA vocal performance with no instrumental accompaniment.
2.adjectiveSinging solely or mainly without instrumental accompaniment.
"An a cappella group sang during the wedding reception."
3.adjectiveRelated to a form of purely vocal music mostly associated with American college performance groups.
4.adjectiveAlla breve.
5.adverbIn a manner of a choir with no instrumental accompaniment; literally, "in the style of the (Sistine) Chapel (in Rome)", such as a musical Mass done a cappella.
"Groups of teens singing a cappella on street corners got recording contracts."