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]

Moby Thesaurus II by Grady Ward, 1.0:

127 Moby Thesaurus words for "decrescendo":
   a cappella, abbandono, accrescendo, adagietto, adagio, affettuoso,
   agilmente, agitato, allegretto, allegro, amabile, amoroso, andante,
   andantino, appassionatamente, appassionato, barely audible,
   brillante, capriccioso, catabasis, collapse, con affetto,
   con agilita, con agitazione, con amore, contractive, crash,
   crescendo, deceleration, declension, decline, decline and fall,
   declining, decreasing, decreasingly, decrescent, deliquescent, dim,
   diminishing, diminishingly, diminuendo, dimness, distant, dive,
   dolce, downtrend, downturn, drop, dwindling, ebb, ever less, faint,
   faint-voiced, faintness, fall, feeble, feebleness, flatness, forte,
   fortissimo, gentle, gentleness, half-heard, indistinct,
   indistinctness, lamentabile, languishing, lapse, larghetto,
   larghissimo, largo, legato, leggiero, less, less and less,
   lessening, low, lowness, marcando, morendo, murmured, on the wane,
   parlando, pianissimo, piano, pizzicato, plunge, prestissimo,
   presto, rallentando, reductive, remission, retreat, ritardando,
   ritenuto, scarcely heard, scherzando, scherzo, scherzoso, slowdown,
   slump, soft, soft-sounding, soft-voiced, softness, sordo,
   sotto voce, spiccato, staccato, stretto, subaudibility, subaudible,
   subdued, subduedness, subsidence, subsiding, tremolando, tremoloso,
   trillando, unclear, unclearness, wane, waning, weak, weak-voiced,
   weakness, whispered