Source : Webster's Revised Unabridged Dictionary (1913)

Uncommon \Un*com"mon\, a.
   Not common; unusual; infrequent; rare; hence, remarkable;
   strange; as, an uncommon season; an uncommon degree of cold
   or heat; uncommon courage.

   Syn: Rare; scarce; infrequent; unwonted. -- {Un*com"mon*ly},
        adv. -- {Un*com"mon*ness}, n.

Source : WordNet®

uncommon
     adj 1: not common or ordinarily encountered; unusually great in
            amount or remarkable in character or kind; "uncommon
            birds"; "frost and floods are uncommon during these
            months"; "doing an uncommon amount of business"; "an
            uncommon liking for money"; "he owed his greatest debt
            to his mother's uncommon character and ability" [ant:
            {common}]
     2: marked by an uncommon quality; especially superlative or
        extreme of its kind; "what is so rare as a day in
        June"-J.R.Lowell; "a rare skill"; "an uncommon sense of
        humor"; "she was kind to an uncommon degree" [syn: {rare}]