Source : Webster's Revised Unabridged Dictionary (1913)

Accelerando \Ac*cel`er*an"do\, a. [It.] (Mus.)
   Gradually accelerating the movement.

Source : WordNet®

accelerando
     adj : gradually increasing in tempo
     n : a gradually increasing tempo of music; "my ear will not
         accept such violent accelerandos"
     adv : with increasing speed; "here you must play accelerando"