The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

diminuendo
    adj 1: gradually decreasing in volume [syn: decrescendo,
           diminuendo]
    n 1: (music) a gradual decrease in loudness [syn: decrescendo,
         diminuendo]