Before the National Institutes of Health began their research in the 1980's, the only definition of dyslexia was an exclusionary one. If a child's difficulty with reading could not be explained by low intelligence, poor eyesight, poor hearing, inadequate educational opportunities, or any other problem, then the child must be dyslexic.
That definition was not satisfactory to parents, teachers, or researchers. So here are three different definitions in use today.
Dyslexia is an inherited condition that makes it extremely difficult to read, write, and spell in your native language—despite at least average intelligence.
Dyslexia is a neurologically-based, often familial, disorder which interferes with the acquisition and processing of language. Varying in degrees of severity, it is manifested by difficulties in receptive and expressive language, including phonological processing, in reading, writing, spelling, handwriting, and sometimes in arithmetic.
Dyslexia is not the result of lack of motivation, sensory impairment, inadequate instructional or environmental opportunities, or other limiting conditions, but may occur together with these conditions.
Although dyslexia is lifelong, individuals with dyslexia frequently respond successfully to timely and appropriate intervention.
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.