What Is Dyslexia?
Dr. Gavin Reid summarised dyslexia as  “a processing difference experienced by people of all ages, often characterised by difficulties in literacy, it can affect other cognitive areas such as memory, speed of processing, time management, co-ordination and directional aspects. There may be visual and phonological difficulties and there is usually some discrepancy in performances in different areas of learning. It is important that the individual differences and learning styles are acknowledged since these will affect outcomes of assessment and learning. It is also important to consider the learning and work context as the nature of the difficulties associated with dyslexia may be more pronounce in some learning situations.” (2002)

In England in recent court cases the following  definition has been accepted: "Dyslexia is evident when accurate and fluent word reading and/or spelling develops incompletely or with great difficulty. This focuses on literacy at the 'word level' and implies that the problem is severe and persistent despite appropriate learning opportunities. It provides the basis of a staged process of assessment through teaching." (British Psychological Society 1999)

The definition adopted by the International Dyslexia Board, and also used by the  and the US National Institute of Child Health and Human Development (NICHD) (2002) is: "Dyslexia is a specific learning disability that is neurological in origin. It is characterised by difficulties with accurate and/or fluent word recognition and by poor spelling and decoding abilities. These difficulties typically result from a deficit in the phonological component of language that is ofter unexpected in relation to other cognitive abilities and the provision of effective classroom instruction. Secondary consequences may include problems in reading comprehension and reduced reading experience that can impede growth of vocabulary and background knowledge." 

Margaret Snowling Professor of Psychology at the University of York in her book Dyslexia Second Edition 2000 (Blackwell  Publishing) considered definitions of dyslexia and concluded that:

“A substantial minority of children have specific difficulty acquiring literacy skills, and these difficulties can be considered ‘unexpected’ because they occur in otherwise bright and able children who master other tasks well. These children are sometimes called dyslexic, and current estimates suggest that between 3 and 10 per cent of the population are so affected.”

T.R. Miles Professor Emeritus of Psychology University of Wales, who has written prolifically on the subject, summarised dyslexia:

"For neurological reasons - connected with deficiencies in the magnocellular system - there may be an anomaly of development which sometimes gives rise to an unusual balance of skill. This anomaly is sometimes but not always, the result of hereditary factors. Reasoning is not affected, and in some areas such as art and engineering there may be exceptional talent. There are weaknesses, however, which may show themselves in spoken language. Many of these weaknesses are overcome by practice and suitable training, but the processing of symbolic material at speed remains difficult." (1993)

When I first considered dyslexia I thought of two children off to play tennis. The first had a bag to carry the balls. The second had too many balls and nothing to carry them in. The first child quickly arrived at the courts. The second child dropped some of the balls but got there in the end. The first child had an advantage and got to the court first. The second child  is not necessarily the worst player. Dyslexics have difficulty holding information they are working with. Once this information is processed the dyslexic has no special difficulties working things out.

References:

British Psychological Society 1999 http://www.psychtesting.org.uk/hotissues.asp?id=83 (Assessed April 2006)

International Dyslexia Association http://www.interdys.org/ (Assessed April 2006)

T. R. Miles Dyslexia - The Pattern of Difficulties Second Edition  (1993) Whurr Publishers  

Gavin Reid Dyslexia - A Practioner's Handbook Third Edition (2004) Wiley

Margaret Snowling Dyslexia Second Edition 2000 Balckwell Publishing

US National Institute of Child Health and Human Development   http://www.nih.gov/science/ (Assessed April 2006)