The International Dyslexia Association defines “dyslexia” in the following way:

Dyslexia is a specific learning disability that is neurobiological in origin. It is characterized 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 often 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.

-Adopted by the International Dyslexia Association Board of Directors, November 12, 2002

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Let's Break Down This Definition:
Specific learning disability – research has indicated specific cognitive characteristics related to dyslexia.

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…that is neurological in origin – dyslexia results from differences in how the brain processes information. Specifically, functional brain imaging has demonstrated a failure of the left hemisphere posterior brain systems to function properly during reading.

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[Source: A Definition of Dyslexia by G. Reid Lyon, Sally E. Shaywitz and Bennett A. Shaywitz; Annuals of Dyslexia, Volume 53, 2003]

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