The Collaborative International Dictionary of English v.0.48:

Appoggiatura \Ap*pog`gia*tu"ra\, n. [It., fr. appogiarre to
   lean, to rest; ap- (L. ad) + poggiare to mount, ascend,
   poggio hill, fr. L. podium an elevated place.] (Mus.)
   A passing tone preceding an essential tone, and borrowing the
   time it occupies from that; a short auxiliary or grace note
   one degree above or below the principal note unless it be of
   the same harmony; -- generally indicated by a note of smaller
   size, as in the illustration above. It forms no essential
   part of the harmony.
   [1913 Webster]

WordNet (r) 3.0 (2006):

appoggiatura
    n 1: an embellishing note usually written in smaller size [syn:
         grace note, appoggiatura, acciaccatura]