The Collaborative International Dictionary of English v.0.48:

Accessibility \Ac*cess`i*bil"i*ty\, n. [L. accessibilitas: cf.
   F. accessibilit['e].]
   The quality of being accessible, or of admitting approach;
   receptibility. --Langhorne.
   [1913 Webster]

WordNet (r) 3.0 (2006):

accessibility
    n 1: the quality of being at hand when needed [syn: handiness,
         accessibility, availability, availableness] [ant:
         inaccessibility, unavailability]
    2: the attribute of being easy to meet or deal with [syn:
       approachability, accessibility] [ant:
       unapproachability]