The Collaborative International Dictionary of English v.0.48:

Loneliness \Lone"li*ness\, n.
   1. The condition of being lonely; solitude; seclusion.
      [1913 Webster]

   2. The state of being unfrequented by human beings; as, the
      loneliness of a road.
      [1913 Webster]

   3. Love of retirement; disposition to solitude.
      [1913 Webster]

            I see
            The mystery of your loneliness.       --Shak.
      [1913 Webster]

   4. A feeling of depression resulting from being alone.
      [1913 Webster]

   Syn: Solitude; seclusion. See Solitude.
        [1913 Webster]

WordNet (r) 3.0 (2006):

loneliness
    n 1: the state of being alone in solitary isolation [syn:
         loneliness, solitariness]
    2: sadness resulting from being forsaken or abandoned [syn:
       forlornness, loneliness, desolation]
    3: a disposition toward being alone [syn: aloneness,
       loneliness, lonesomeness, solitariness]

Moby Thesaurus II by Grady Ward, 1.0:

20 Moby Thesaurus words for "loneliness":
   alienation, aloneness, aloofness, celibacy, detachment, isolation,
   keeping apart, loneness, lonesomeness, moving apart, privacy,
   seclusion, separateness, sequestration, single blessedness,
   singleness, solitariness, solitude, splendid isolation,
   withdrawal