Source : Webster's Revised Unabridged Dictionary (1913)

Diminuendo \Di*min`u*en"do\, adv. [It., p. pr. of diminuere to
   diminish.] (Mus.)
   In a gradually diminishing manner; with abatement of tone;
   decrescendo; -- expressed on the staff by Dim., or Dimin., or
   the sign.

Source : WordNet®

diminuendo
     adj : gradually decreasing in volume [syn: {decrescendo}]
     n : (music) a gradual decrease in loudness [syn: {decrescendo}]