I just listened to a recording of a keynote address given by Dr. Joseph Torgesen of the Florida Center for Reading Research at the 2nd Annual Reading Conference for the Utah Branch of the International Dyslexia Association.  He shared an interesting science-based definition of dyslexia: 

“Dyslexia is a specific learning disability that is neurobiological in origin. It is characterized by difficulties with accurate and/or fluent word recognition and by poor spelling and decoding abilities. These difficulties typically result from a deficit in the phonological component of language that is often unexpected in relation to other cognitive abilities and the provision of effective classroom instruction...
 
“Secondary consequences may include problems in reading comprehension and reduced reading experience that can impede growth of vocabulary and background knowledge.”
NOTE:  Emphasis added by Torgeson. 
(Source: Lyon, G. R. & Shaywitz, S. E. (2003). A definition of dyslexia. Annals of Dyslexia, 53, 1-14.)