A Cappella : a cap·pel·la : ˌä-kə-ˈpe-lə
Used as an adverb or adjective
Choral or vocal music performed without instrumental accompaniment.
The first known use of a cappella was in 1975, borrowed from the Italian a cappella to mean "in chapel or choir style." (Source: Merriam-Webster)
Sentence Example: The band decided to perform a song a cappella to stand out to the judges.