Webster's English Dictionary

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visibility

vis.i.bil.i.ty \.viz-*-'bil-*t-e-\ n 1: the quality or state of being 
   visible  2a: the degree of clearness of the atmosphere; specif : the 
   greates t distance toward the horizon that prominent objects can be 
   identified visually with the naked eye 2b: capability of affording an 
   unobstructed view  3: a measure of the ability of radiant energy to evoke 
   visual sensation