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"

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "a cappella":
   abbandono, accrescendo, affettuoso, agilmente, agitato, amabile,
   amoroso, appassionatamente, appassionato, brillante, capriccioso,
   con affetto, con agilita, con agitazione, con amore, crescendo,
   decrescendo, diminuendo, dolce, forte, fortissimo, lamentabile,
   leggiero, morendo, parlando, pianissimo, piano, pizzicato,
   scherzando, scherzoso, sordo, sotto voce, spiccato, staccato,
   stretto, tremolando, tremoloso, trillando