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

decelerate \decelerate\ v. i.
   1. 1 to reduce speed; as, the car decelerated. opposite of
      accelerate.

   syn: slow, slow down, slow up, retard.
        wordnet 1.5
see also:
accelerate 

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

decelerate \decelerate\ v. t.
   to cause to reduce speed. opposite of accelerate.

   syn: slow down.
        wordnet 1.5
see also:
accelerate 

[3] : WordNet (r) 2.0

decelerate
     v 1: lose velocity; move more slowly; "the car decelerated" syn:
           slow, slow down, slow up, retard ant: accelerate
     2: reduce the speed of; "he slowed down the car" syn: slow
        down ant: accelerate
see also:
slow slow down slow up retard accelerate slow down 

Decelerate >> D Words
Decelerate, definition of term: Decelerate
decelerate_pag1.html