The Collaborative International Dictionary of English v.0.48:

Acceleration \Ac*cel`er*a"tion\, n. [L. acceleratio: cf. F.
   acc['e]l['e]ration.]
   The act of accelerating, or the state of being accelerated;
   increase of motion or action; as, a falling body moves toward
   the earth with an acceleration of velocity; -- opposed to
   retardation.
   [1913 Webster]

         A period of social improvement, or of intellectual
         advancement, contains within itself a principle of
         acceleration.                            --I. Taylor.
   [1913 Webster] (Astr. & Physics.)

   Acceleration of the moon, the increase of the moon's mean
      motion in its orbit, in consequence of which its period of
      revolution is now shorter than in ancient times.

   Acceleration and retardation of the tides. See Priming
      of the tides, under Priming.

   Diurnal acceleration of the fixed stars, the amount by
      which their apparent diurnal motion exceeds that of the
      sun, in consequence of which they daily come to the
      meridian of any place about three minutes fifty-six
      seconds of solar time earlier than on the day preceding.


   Acceleration of the planets, the increasing velocity of
      their motion, in proceeding from the apogee to the perigee
      of their orbits.
      [1913 Webster]

WordNet (r) 3.0 (2006):

acceleration
    n 1: an increase in rate of change; "modern science caused an
         acceleration of cultural change" [ant: deceleration,
         retardation, slowing]
    2: the act of accelerating; increasing the speed [syn:
       acceleration, quickening, speedup] [ant:
       deceleration]
    3: (physics) a rate of increase of velocity [ant:
       deceleration]

Moby Thesaurus II by Grady Ward, 1.0:

40 Moby Thesaurus words for "acceleration":
   accelerando, aggravation, beefing-up, blowing up, blowup,
   concentration, condensation, consolidation, deepening, double time,
   double-quick, double-quick time, drive, enhancement, exacerbation,
   exaggeration, explosion, festination, forced march, forwarding,
   getaway, hastening, heating-up, heightening, hurrying, impetus,
   information explosion, intensification, magnification, pickup,
   population explosion, quickening, redoubling, reinforcement,
   speeding, speedup, step-up, strengthening, thrust, tightening