Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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