The Collaborative International Dictionary of English v.0.48:

Retardation \Re`tar*da"tion\, n. [L. retardatio: cf. F.
   retardation.]
   1. The act of retarding; hindrance; the act of delaying; as,
      the retardation of the motion of a ship; -- opposed to
      acceleration.
      [1913 Webster]

            The retardations of our fluent motion. --De Quinsey.
      [1913 Webster]

   2. That which retards; an obstacle; an obstruction.
      [1913 Webster]

            Hills, sloughs, and other terrestrial retardations.
                                                  --Sir W.
                                                  Scott.
      [1913 Webster]

   3. (Mus.) The keeping back of an approaching consonant chord
      by prolonging one or more tones of a previous chord into
      the intermediate chord which follows; -- differing from
      suspension by resolving upwards instead of downwards.
      [1913 Webster]

   4. The extent to which anything is retarded; the amount of
      retarding or delay.
      [1913 Webster]

   Retardation of the tide.
      (a) The lunitidal interval, or the hour angle of the moon
          at the time of high tide any port; the interval
          between the transit of the moon and the time of high
          tide next following.
      (b) The age of the tide; the retard of the tide. See under
          Retard, n.
          [1913 Webster]

WordNet (r) 3.0 (2006):

retardation
    n 1: a decrease in rate of change; "the deceleration of the arms
         race" [syn: deceleration, slowing, retardation] [ant:
         acceleration]
    2: the extent to which something is delayed or held back
    3: any agent that retards or delays or hinders; "flame-
       retardant" [syn: retardant, retardent, retardation]
    4: lack of normal development of intellectual capacities [syn:
       retardation, mental retardation, backwardness,
       slowness, subnormality]
    5: the act of slowing down or falling behind [syn: slowdown,
       lag, retardation]

Moby Thesaurus II by Grady Ward, 1.0:

136 Moby Thesaurus words for "retardation":
   afterthought, amentia, arrest, arrestation, arrested development,
   arrestment, backwardness, bind, blithering idiocy, block, blockage,
   blocking, bureaucratic delay, check, clogging, closing up, closure,
   constraint, constriction, control, cooling, cooling down,
   cooling off, cramp, cretinism, curb, curtailment, deceleration,
   delay, delayage, delayed reaction, detainment, drag, dragging,
   ease-off, ease-up, fixation, flagging, foot-dragging,
   half-wittedness, halt, hampering, hang-up, hindering, hindrance,
   holdback, holdup, idiocy, idiotism, imbecility, impediment,
   infantilism, inhibition, injunction, insanity, interdict,
   interference, interim, interruption, jam, lag, lagging,
   legal restraint, let, letdown, letup, logjam, mental defectiveness,
   mental deficiency, mental handicap, mental retardation,
   minus acceleration, mongolianism, mongolism, mongoloid idiocy,
   monopoly, moratorium, moronism, moronity, negativism,
   nuisance value, obstruction, obstructionism, occlusion, opposition,
   paperasserie, pause, profound idiocy, prohibition, protection,
   protectionism, protective tariff, rationing, red tape, red-tapeism,
   red-tapery, rein, repression, reprieve, resistance, respite,
   restraint, restraint of trade, restriction, retardance, retardment,
   retrenchment, self-control, setback, simple-wittedness,
   simplemindedness, simpleness, simplicity, slack-up, slackening,
   slow-up, slowdown, slowing, slowing down, slowness, slowup,
   squeeze, stay, stay of execution, stop, stoppage, stranglehold,
   stricture, subnormality, suppression, suspension, tariff wall,
   thought control, tie-up, time lag, wait