The BDA definition is as follows: "Dyslexia is best described as a combination of abilities and difficulties that affect the learning process in one or more of reading, spelling, writing. Accompanying weaknesses may be identified in areas of speed of processing, short-term memory, sequencing and organisation, auditory and/or visual perception, spoken language and motor skills. It is particularly related to mastering and using written language, which may include alphabetic, numeric and musical notation. Some dyslexics have outstanding creative skills. Others have strong oral skills. Some have no outstanding talents. They all have strengths. Dyslexia can occur despite normal intellectual ability and teaching. It is independent of socio-economic or language background."(The Dyslexia Handbook 2002)

The International Dyslexia Association says: "Dyslexia is a neurologically-based, often familial, disorder which interferes with the acquisition and processing of language. Varying in degrees of severity, it is manifested by difficulties in receptive and expressive language, including phonological processing, in reading, writing, spelling, handwriting, and sometimes in arithmetic."

The first diagnosis of developmental dyslexia appeared in The British Medical Journal in 1896: "A Case of Congenital Word Blindness" by W. Pringle Morgan. It was an account of a 14 year old boy, Percy: "...in spite of this laborious and persistent training, he can only with difficulty spell out words of one syllable". "The schoolmaster who taught him for some years says that he would be the smartest lad in the school if the instruction were entirely oral." The very first reference may be Valerius Maximus (c30 AD), who writes about an Athenian scholar who "lost his memory of letters" after being struck in the head with a stone and Pliny (23-79 AD) who says of the same man that "with the stroke of a stone, (he) fell presently to forget his letters only, and could read no more; otherwise his memory served him well enough" (in Benton and Joynt 1960)."