The Collaborative International Dictionary of English v.0.48:

Solitude \Sol"i*tude\, n. [F., from L. solitudo, solus alone.
   See Sole, a.]
   1. state of being alone, or withdrawn from society; a lonely
      life; loneliness.
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            Whosoever is delighted with solitude is either a
            wild beast or a god.                  --Bacon.
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            O Solitude! where are the charms
            That sages have seen in thy face?     --Cowper.
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   2. Remoteness from society; destitution of company;
      seclusion; -- said of places; as, the solitude of a wood.
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            The solitude of his little parish is become matter
            of great comfort to him.              --Law.
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   3. solitary or lonely place; a desert or wilderness.
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            In these deep solitudes and awful cells
            Where heavenly pensive contemplation dwells. --Pope.
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   Syn: Syn. Loneliness; soitariness; loneness; retiredness;
        recluseness. -- Solitude, Retirement, Seclusion,
        Loneliness.

   Usage: Retirement is a withdrawal from general society,
          implying that a person has been engaged in its scenes.
          Solitude describes the fact that a person is alone;
          seclusion, that he is shut out from others, usually by
          his own choice; loneliness, that he feels the pain and
          oppression of being alone. Hence, retirement is
          opposed to a gay, active, or public life; solitude, to
          society; seclusion, to freedom of access on the part
          of others; and loneliness, enjoyment of that society
          which the heart demands.
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                O blest retirement, friend to life's decline.
                                                  --Goldsmith.
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                Such only can enjoy the country who are capable
                of thinking when they are there; then they are
                prepared for solitude; and in that [the country]
                solitude is prepared for them.    --Dryden.
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                It is a place of seclusion from the external
                world.                            --Bp. Horsley.
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                These evils . . . seem likely to reduce it [a
                city] ere long to the loneliness and the
                insignificance of a village.      --Eustace.
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WordNet (r) 3.0 (2006):

solitude
    n 1: a state of social isolation [syn: solitude, purdah]
    2: the state or situation of being alone
    3: a solitary place

Moby Thesaurus II by Grady Ward, 1.0:

26 Moby Thesaurus words for "solitude":
   alienation, aloneness, aloofness, celibacy, confinement,
   detachment, emptiness, isolation, keeping apart, loneliness,
   loneness, lonesomeness, moving apart, privacy, quarantine,
   remoteness, retirement, seclusion, separateness, sequestration,
   single blessedness, singleness, solitariness, splendid isolation,
   wilderness, withdrawal