The Collaborative International Dictionary of English v.0.48:

Isolation \I`so*la"tion\ ([imac]`s[-o]*l[=a]"sh[u^]n), n. [Cf.
   F. isolation.]
   The act of isolating, or the state of being isolated;
   insulation; separation; loneliness. --Milman.
   [1913 Webster]

WordNet (r) 3.0 (2006):

isolation
    n 1: a state of separation between persons or groups
    2: a feeling of being disliked and alone
    3: the act of isolating something; setting something apart from
       others [syn: isolation, closing off]
    4: (psychiatry) a defense mechanism in which memory of an
       unacceptable act or impulse is separated from the emotion
       originally associated with it
    5: a country's withdrawal from international politics; "he
       opposed a policy of American isolation"