Dyslexia is a learning disability that manifests primarily as a difficulty with written language, particularly with reading and spelling. It is separate and distinct from reading difficulties resulting from other causes, such as a non-neurological deficiency with vision or hearing, or from poor or inadequate reading instruction.

Evidence suggests that dyslexia results from differences in how the brain processes written and/or verbal language. Although dyslexia is the result of a neurological difference, it is not an intellectual disability. Dyslexia occurs at all levels of intelligence; sub-average, average, above average, and highly gifted.

A University of Hong Kong study argues that dyslexia affects different structural parts of children's brains depending on the language which the children read. The study focused on comparing children that were raised reading English and children raised reading Chinese. Using MRI technology researchers found that the children reading English used a different part of the brain than those reading Chinese. Researchers were surprised by this discovery and hope that the findings will help lead them to any neurobiological cause for dyslexia.

The word dyslexia comes from the Greek words δυσ- dys- ("impaired") and λέξις lexis ("word"). People with dyslexia are called dyslexic or dyslectic.