Source : Webster's Revised Unabridged Dictionary (1913)

Vox angelica \Vox` an*gel"i*ca\ [L. angelica angelic.] (Music)
   An organ stop of delicate stringlike quality, having for each
   finger key a pair of pipes, of which one is tuned slightly
   sharp to give a wavy effect to their joint tone.

Source : WordNet®

vox angelica
     n : an organ stop producing a gentle tremolo effect [syn: {voix
         celeste}]