Source : Webster's Revised Unabridged Dictionary (1913)

Unusual \Un*u"su*al\, a.
   Not usual; uncommon; rare; as, an unusual season; a person of
   unusual grace or erudition. -- {Un*u"su*al*ly}, adv. --
   {Un*u"su*al*ness}, n.

Source : WordNet®

unusualness
     n : uncommonness by virtue of being unusual [ant: {usualness}]