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]

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]

WordNet (r) 3.0 (2006):

decelerate
    v 1: lose velocity; move more slowly; "The car decelerated"
         [syn: decelerate, slow, slow down, slow up,
         retard] [ant: accelerate, quicken, speed, speed
         up]
    2: reduce the speed of; "He slowed down the car" [syn:
       decelerate, slow down] [ant: accelerate, speed,
       speed up]