Source : Webster's Revised Unabridged Dictionary (1913)

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.