Dyslexia is typically “diagnosed” by a psychologist and “treated” by educators (though I personally prefer non-medical language like “identified” and “supported”). As such, you will find it defined in two places: education guidelines and medical diagnostic manuals.
Check your local education department to see if they have special education guidelines for dyslexia diagnosis and support.
Here in Ontario, new draft guidelines have dropped the word dyslexia, however it is broadly defined as a learning disability under the “Communicational” exceptionality:
The world’s two main medical diagnostic guides define dyslexia as follows:
DSM-5 (Diagnostic and Statistical Manual of Mental Disorders)
Dyslexia is categorized as a “specific learning disorder” in the DSM-5 (published in 2013), which is the handbook used by health care professionals in the United States and internationally. It is considered the authoritative guide to the diagnosis of “mental disorders” (though it is troubling that dyslexia is categorized as a “mental illness“).
DSM-5 was a change from the previous edition, which included dyslexia as a distinct category:
“Specific Learning Disorder” has become the umbrella term for mathematics, reading, and written expression disorders in the updated DSM-5. The DSM-IV previously classified these as separate diagnoses. Instead, these disorders are now housed under one diagnosis with added specifiers (e.g., specific learning disorder with impaired reading).” (source: https://psychcentral.com/disorders/specific-learning-disorder/)
ICD-II (International Classification of Diseases)
Dyslexia is categorized as a “Developmental learning disorder with impairment in reading” in the new ICD-II guidelines (published December 2017). The criteria are part of a highly anticipated update to the “International Classification of Diseases,” a diagnostic manual produced by the World Health Organization (WHO).