Source : Webster's Revised Unabridged Dictionary (1913)

Rallentando \Ral"len*tan"do\, a. [It.] (Mus.)
   Slackening; -- a direction to perform a passage with a
   gradual decrease in time and force; ritardando.

Source : WordNet®

rallentando
     adj : gradually decreasing in tempo [syn: {ritardando}, {ritenuto},
            {rit.}]
     adv : slowing down; "this passage should be played rallentando"