a cappella

American Heritage® Dictionary of the English Language, Fourth Edition

1. adv. Music Without instrumental accompaniment.

Century Dictionary and Cyclopedia

1. n. In the style of church or chapel music. Applied to compositions sung without instrumental accompaniment, or with an accompaniment in unison with the vocal part: as, a mass a cappella.
2. n. In music, noting a species of time which employs four minims or half-notes in each measure.

Wiktionary

1. adv. Performed by a soloist or choir with no instrumental accompaniment; literally "in the style of the (Sistine) Chapel (in Rome)"; such as a musical Mass done a cappella.
2. adv. A time indication, equivalent to alla breve.
3. adv. Performed vocally without instrumental accompaniment.
4. adj. sung without instrumental accompaniment
5. adj. singing solely or mainly without instrumental accompaniment.
6. adv. Performed by a soloist or choir with no instrumental accompaniment; literally "in the style of the (Sistine) Chapel (in Rome)"; such as a musical Mass done a cappella.
7. adv. A time indication, equivalent to alla breve.
8. adv. Performed vocally without instrumental accompaniment.
9. adj. sung without instrumental accompaniment
10. adj. singing solely or mainly without instrumental accompaniment.

GNU Webster's 1913

1. In church or chapel style; -- said of compositions sung in the old church style, without instrumental accompaniment.
2. A time indication, equivalent to alla breve.

WordNet 3.0

1. adv. without musical accompaniment
2. adj. sung without instrumental accompaniment