A cappella \A cap*pel"la\ [It. See Chapel.]

(Mus.)
   (a) In church or chapel style; -- said of compositions sung in the old church style, without instrumental accompaniment; as, a mass a capella, i. e., a mass purely vocal.
   (b) A time indication, equivalent to alla breve.

Source: Webster's Revised Unabridged Dictionary (1913)