Dyslexia is a reading disorder. It is a specific learning difficulty in which a person has persistent trouble with accurate and fluent word recognition, with decoding written words, and with spelling.

The underlying difficulty is phonological: the learner struggles to connect written letters with the sounds they represent and to break spoken words into their component sounds. In practice this shows as slow and effortful reading, frequent misreading of words, confusion between visually similar letters such as b and d, difficulty sounding out unfamiliar words, and poor spelling. Because so much energy goes into decoding the words, comprehension of the passage often suffers even when the learner would have understood the same content if it were read aloud to them.

The defining feature is that this difficulty is unexpected. It occurs in people of normal or high intelligence, with adequate schooling and no sensory impairment, which is exactly why it counts as a specific learning disability rather than a general one. With appropriate teaching - structured phonics instruction, extra time, audio material - people with dyslexia read successfully, and many are notably able in other areas.

Mental disorder is wrong because dyslexia is not a psychiatric illness and is unrelated to intelligence. Behavioural disorder is wrong because it concerns conduct rather than learning, though frustration at school can produce behavioural problems as a secondary effect. Writing disorder is the closest distractor and names a real condition, but that condition is dysgraphia, which affects handwriting and written expression. Dyslexia does often affect spelling, yet its defining domain is reading.

Hence dyslexia is associated with reading disorder.