ADJECTIVE (1)

1. sung without instrumental accompaniment;
- Example: "they sang an a cappella Mass"


ADVERB (1)

1. without musical accompaniment;
- Example: "they performed a cappella"

The Collaborative International Dictionary of English v.0.48:

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.
          [1913 Webster]

WordNet (r) 3.0 (2006):

a cappella
    adv 1: without musical accompaniment; "they performed a
           cappella"
    adj 1: sung without instrumental accompaniment; "they sang an a
           cappella Mass"