Source : Webster's Revised Unabridged Dictionary (1913)

Ritardando \Ri`tar*dan"do\, a. [It.] (Mus.)
   Retarding; -- a direction for slower time; rallentado.

Source : WordNet®

ritardando
     adj : gradually decreasing in tempo [syn: {rallentando}, {ritenuto},
            {rit.}]