The following is the research definition of dyslexia used by the National Institutes of Health and the International Dyslexia Association (Nov. 12, 2002):
Dyslexia is a specific learning disability that is neurological 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.
Dyslexia is not an indicator of low intelligence. In fact, people with dyslexia posses at least average intelligence, and most often surpass such a standard.