Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

appoggiatura
     n : an embellishing note usually written in smaller size [syn: {grace
         note}, {acciaccatura}]
     [also: {appoggiature} (pl)]