The Collaborative International Dictionary of English v.0.48:

Dissociation \Dis*so`ci*a"tion\ (?; 106), n. [L. dissociatio:
   cf. F. dissociation.]
   1. The act of dissociating or disuniting; a state of
      separation; disunion.
      [1913 Webster]

            It will add infinitely dissociation, distraction,
            and confusion of these confederate republics.
                                                  --Burke.
      [1913 Webster]

   2. (Chem.) The process by which a compound body breaks up
      into simpler constituents; -- said particularly of the
      action of heat on gaseous or volatile substances; as, the
      dissociation of the sulphur molecules; the dissociation of
      ammonium chloride into hydrochloric acid and ammonia.
      [1913 Webster]

WordNet (r) 3.0 (2006):

dissociation
    n 1: the act of removing from association
    2: a state in which some integrated part of a person's life
       becomes separated from the rest of the personality and
       functions independently [syn: dissociation,
       disassociation]
    3: (chemistry) the temporary or reversible process in which a
       molecule or ion is broken down into smaller molecules or ions