The Collaborative International Dictionary of English v.0.48:

Accelerate \Ac*cel"er*ate\, v. t. [imp. & p. p. Accelerated;
   p. pr. & vb. n. Accelerating.] [L. acceleratus, p. p. of
   accelerare; ad + celerare to hasten; celer quick. See
   Celerity.]
   1. To cause to move faster; to quicken the motion of; to add
      to the speed of; -- opposed to retard.
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   2. To quicken the natural or ordinary progression or process
      of; as, to accelerate the growth of a plant, the increase
      of wealth, etc.
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   3. To hasten, as the occurence of an event; as, to accelerate
      our departure.
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   Accelerated motion (Mech.), motion with a continually
      increasing velocity.

   Accelerating force, the force which causes accelerated
      motion.                                     --Nichol.
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   Syn: To hasten; expedite; quicken; dispatch; forward;
        advance; further.
        [1913 Webster]

WordNet (r) 3.0 (2006):

accelerate
    v 1: move faster; "The car accelerated" [syn: accelerate,
         speed up, speed, quicken] [ant: decelerate,
         retard, slow, slow down, slow up]
    2: cause to move faster; "He accelerated the car" [syn:
       accelerate, speed, speed up] [ant: decelerate, slow
       down]