The definition of dyslexia is refined every time new research is published. But the British Dyslexia Association's most up-to-date working definition refers to "difficulties which affect the learning process in one or more of reading, spelling and writing".
The definition goes on to give "accompanying weaknesses", including short-term memory, spoken language and motor skills.
The BDA points out dyslexia - said to affect one person in 20 - is independent of social or economic factors or intelligence.
Earlier this year psychologists from the University of Sheffield announced dyslexia was likely to be caused by abnormalities in the parts of the brain that control muscular movements and balance. For the past decade the established view was that the problem stemmed from the parts of the brain controlling language and the understanding of sounds.
Last year, scientists at Yale University published an explanation of how in dyslexics the neural pathway used for reading ended in a different lobe of the brain to non-dyslexics. The findings excited the scientific community because it showed that there was a physical basis for dyslexia.