Dyslexia is a neurological disorder with biochemical and genetic markers. In its most common and apparent form, it is a disability in which a person's reading and/or writing ability is significantly lower than that which would be predicted by his or her general level of intelligence. However, as has been discovered only in the last decade, individuals may read and write perfectly and still have dyslexia.

The term was coined in 1887. People are diagnosed as dyslexic when their reading problems cannot be explained by a lack of intellectual ability, inadequate instruction, or sensory problems such as poor eyesight. Because reading is a complex mental process, dyslexia has many potential causes. From a neurophysiological perspective, dyslexia can be diagnosed by close inspection of the morphology of the brain, usually upon autopsy. Dyslexia is also associated with phonological difficulties, such as enunciation.