The Collaborative International Dictionary of English v.0.48:

Ventriloquism \Ven*tril"o*quism\, n. [See Ventriloquous.]
   The act, art, or practice of speaking in such a manner that
   the voice appears to come, not from the person speaking, but
   from some other source, as from a dummy held by the
   ventriloquist, from the opposite side of the room, from the
   cellar, etc.
   [1913 Webster]

WordNet (r) 3.0 (2006):

ventriloquism
    n 1: the art of projecting your voice so that it seems to come
         from another source (as from a ventriloquist's dummy) [syn:
         ventriloquism, ventriloquy]