Texas Education Code (TEC) §38.003 defines dyslexia and related disorders in the following way:
"Dyslexia" means a disorder of constitutional origin manifested by a difficulty in learning to read, write, or spell, despite conventional instruction, adequate intelligence, and sociocultural opportunity.
"Related disorders" include disorders similar to or related to dyslexia, such as developmental auditory imperception, dysgraphia, specific-developmental dyslexia, developmental dysgraphia, and developmental spelling disability.
TEC §38.003(d)(1)-(2) (1995) http://www.statutes.legis.state.tx.us/Docs/ED/htm/ED.38.htm#38.003
Texas Dyslexia Handbook, 2018 Update https://tea.texas.gov/sites/default/files/2018-Dyslexia-Handbook_Approved_Accomodated_12_11_2018.pdf
The International Dyslexia Association defines "dyslexia" in the following way:
"Dyslexia is a specific learning disability that is neurological in nature. 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."
Adopted by the International Dyslexia Association Board of Directors, November 12, 2002