As defined in the Texas Education Code Section 38.003:
Dyslexia means a disorder of constitutional origin manifested by a difficulty in learning to read, write and/or spell despite conventional instruction, adequate intelligence and sociocultural opportunities.
Related disorders include disorders similar to or related to dyslexia such as developmental auditory imperception, dysphasia, specific developmental dyslexia, developmental dysgraphia, and developmental spelling disability.
As defined by the International Dyslexia Association in November 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."