The Collaborative International Dictionary of English v.0.48:

deceleration \deceleration\ n.
   a decrease in velocity. Opposite of acceleration

   Syn: slowing, retardation.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

deceleration
    n 1: a decrease in rate of change; "the deceleration of the arms
         race" [syn: deceleration, slowing, retardation] [ant:
         acceleration]
    2: (physics) a rate of decrease in velocity [ant:
       acceleration]
    3: the act of decelerating; decreasing the speed; "he initiated
       deceleration by braking" [ant: acceleration, quickening,
       speedup]

Moby Thesaurus II by Grady Ward, 1.0:

71 Moby Thesaurus words for "deceleration":
   arrest, arrestation, catabasis, check, collapse, constraint,
   control, cooling, cooling down, cooling off, crash, curb,
   curtailment, declension, decline, decline and fall, decrescendo,
   delay, detention, diminuendo, dive, downtrend, downturn, drag,
   drop, dwindling, ease-off, ease-up, ebb, fall, flagging, hindrance,
   holdup, inhibition, injunction, interdict, lag, lapse,
   legal restraint, letdown, letup, minus acceleration, monopoly,
   obstruction, plunge, prohibition, protection, protectionism,
   protective tariff, rationing, rein, remission, restraint,
   restraint of trade, retardation, retardment, retreat, retrenchment,
   self-control, setback, slack-up, slackening, slowdown, slowing,
   slowing down, slowup, slump, subsidence, tariff wall,
   thought control, wane