Dyslexia, also known as alexia or developmental reading disorder, is characterized by difficulties learning to read and differing comprehension of language despite normal or above-average intelligence. This includes difficulty with phonological awareness, phonological decoding, processing speed, orthographic coding, auditory short-term memory, language skills and verbal comprehension.
Dyslexia is the most common learning difficulty. Some see dyslexia as distinct from reading difficulties resulting from other causes, such as a non-neurological deficiency with hearing or vision, or poor reading instruction. There are three proposed cognitive subtypes of dyslexia (auditory, visual and attentional), although individual cases of dyslexia are better explained by specific underlying neuropsychological deficits (e.g. attention deficit hyperactivity disorder, a visual processing disorder / visual stress) and co-occurring learning difficulties (e.g. dyscalculia and dysgraphia). Although it is considered to be a receptive (afferent) language-based learning disability, dyslexia also affects one's expressive (efferent) language skills.