"Dyslexia is one of several distinct learning disabilities.  It is a specific language-based disorder of constitutional origin characterized by difficulties in single word decoding, usually reflecting insufficient phonological processing.  These difficulties in single word decoding are often UNEXPECTED in relation to age and other cognitive and academic abilities;  they are not the result of generalized developmental disability or sensory impairment. Dyslexia is manifested by variable difficulty with different forms of language, often including, in addition to problems with reading, a conspicuous problem with acquiring proficiency in writing and spelling."  This definition of dyslexia has been adopted by The International Dyslexia Association and the National Institute of Child Health and Development (NICHD).

The International Dyslexia Association refers to dyslexia as a language based learning disability. It is the most common form of learning disability. Individuals with dyslexia usually have difficulty with either receptive oral language skills, expressive oral language skills, reading, spelling, or written expression. Dyslexia varies in degrees of severity. The prognosis depends on the severity of the disability, specific patterns of strengths and weaknesses with the individual, and the appropriateness of the intervention. It is not a result of lack of motivation, sensory impairment, inadequate instruction, environmental opportunities, low intelligence, or other limiting conditions. It is a condition which is neurologically based and often appears in families. Individuals with dyslexia respond successfully to timely and appropriate intervention.