The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

   Syn: Rare; scarce; infrequent; unwonted.
        [1913 Webster] -- Un*com"mon*ly, adv. --
        Un*com"mon*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

uncommonness
    n 1: extraordinariness as a consequence of being marked by an
         uncommon or superlative quality
    2: extraordinariness as a consequence of being rare and seldom
       encountered [ant: commonness, commonplaceness,
       everydayness]