[1] : The Collaborative International Dictionary of English v.0.48

rallentando \ral`lentan"do\ rauml`le^ntaumn"d-o, a.
   it. mus.
   slackening; -- a direction to perform a passage with a
   gradual decrease in time and force; ritardando.
   1913 webster

[2] : WordNet (r) 2.0

rallentando
     adj : gradually decreasing in tempo syn: ritardando, ritenuto,
            rit.
     adv : slowing down; "this passage should be played rallentando"
see also:
ritardando ritenuto rit. 

Rallentando >> R Words
Rallentando, definition of term: Rallentando
rallentando_pag1.html