Source : Webster's Revised Unabridged Dictionary (1913)

Unobtrusive \Un`ob*tru"sive\, a.
   Not obtrusive; not presuming; modest. -- {Un`ob*tru"sive*ly},
   adv. -- {Un`ob*tru"sive*ness}, n.

Source : WordNet®

unobtrusiveness
     n : the quality of not sticking out in an unwelcome way [ant: {obtrusiveness}]