“Dyslexia is a disturbance in certain linguistic functions of critical importance for a productive use of the alphabetic principle when written language is coded. The disturbance is primarily expressed as difficulties in achieving an automatized word recognition during reading. It is also clearly manifested in poor spelling. The dyslexic disturbance often runs in families, and there are reasons to assume that a genetic disposition is involved. A characteristic feature of dyslexia is that it tends to persist. Even though reading sometimes can reach an acceptable level, the problems related to spelling remain.”

More recently the International Dyslexia Association and the National Institute of Child Health and Development (US) (2003) defined dyslexia in a similar way:
“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 impede growth of vocabulary and background knowledge.“