Source : Webster's Revised Unabridged Dictionary (1913)

Readability \Read`a*bil"i*ty\, n.
   The state of being readable; readableness.

Source : WordNet®

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