The Collaborative International Dictionary of English v.0.48:

dyslexia \dys*lex"i*a\ (d[i^]s*l[e^]ks"[-e]*[.a]) n.
   Any of various reading disorders caused by a damaged or
   congenitally faulty structure within the central nervous
   system, and causing an impairment of the ability to interpret
   spatial relationships or to integrate auditory and visual
   information. A common example is when letter sequences are
   interpreted as inverted in order, as in bat/tab. --RHUD
   [PJC]

WordNet (r) 3.0 (2006):

dyslexia
    n 1: impaired ability to learn to read