The Collaborative International Dictionary of English v.0.48:

Sforzando \Sfor*zan"do\, Sforzato \Sfor*za"to\, a. [It.
   sforzando, p. pr., and sforzato, p. p. of sforzare to force.]
   (Mus.)
   Forcing or forced; -- a direction placed over a note, to
   signify that it must be executed with peculiar emphasis and
   force; -- marked fz (an abbreviation of forzando), sf, sfz,
   or ?.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sforzando
    n 1: an accented chord
    2: (music) a notation written above a note and indicating that
       it is to be played with a strong initial attack