Source : Webster's Revised Unabridged Dictionary (1913)

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}.

            The retardations of our fluent motion. --De Quinsey.

   2. That which retards; an obstacle; an obstruction.

            Hills, sloughs, and other terrestrial retardations.
                                                  --Sir W.
                                                  Scott.

   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.

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

   {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.

Source : WordNet®

retardation
     n 1: a decrease in speed; "the deceleration of the arms race"
          [syn: {deceleration}, {slowing}] [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}]
     4: lack of normal development of intellectual capacities [syn:
        {mental retardation}, {backwardness}, {slowness}, {subnormality}]
     5: the act of slowing down or falling behind [syn: {slowdown},
        {lag}]