Dyslexia
If you are speaking with schools or looking at government guidance in the UK, you will likely encounter the definition set out by the Rose Report (2009). It remains the benchmark for education and states:
"Dyslexia is a learning difficulty that primarily affects the skills involved in accurate and fluent word reading and spelling. Characteristic features of dyslexia are difficulties in phonological awareness, verbal memory and verbal processing speed. Dyslexia occurs across the range of intellectual abilities."
This definition is important because it highlights that dyslexia is independent of intelligence. A person can be highly gifted, creative, and articulate, yet still struggle with the mechanics of reading and writing.