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]