The Collaborative International Dictionary of English v.0.48:

Decrescendo \De`cres*cen"do\, a. & adv. [It.] (Mus.)
   With decreasing volume of sound; -- a direction to
   performers, either written upon the staff (abbreviated Dec.,
   or Decresc.), or indicated by the sign.
   [1913 Webster]

WordNet (r) 3.0 (2006):

decrescendo
    adj 1: gradually decreasing in volume [syn: decrescendo,
           diminuendo]
    n 1: (music) a gradual decrease in loudness [syn: decrescendo,
         diminuendo]
    v 1: grow quieter; "The music decrescendoes here" [ant:
         crescendo]