The Collaborative International Dictionary of English v.0.48:

Readability \Read`a*bil"i*ty\ (r[=e]d`[.a]*b[i^]l"[i^]*t[y^]),
   n.
   The state of being readable; readableness.
   [1913 Webster]

WordNet (r) 3.0 (2006):

readability
    n 1: the quality of written language that makes it easy to read
         and understand
    2: a quality of writing (print or handwriting) that can be
       easily read [syn: legibility, readability] [ant:
       illegibility]