The Collaborative International Dictionary of English v.0.48:

Ritardando \Ri`tar*dan"do\, a. [It.] (Mus.)
   Retarding; -- a direction for slower time; rallentado.
   [1913 Webster]

WordNet (r) 3.0 (2006):

ritardando
    adj 1: gradually decreasing in tempo [syn: rallentando,
           ritardando, ritenuto, rit.]