There are number of different definitions of dyslexia. The following is the definition given by the British Dyslexia Association (BDA).
Dyslexia is a specific learning difficulty that mainly affects the development of literacy and language related skills. It is likely to be present at birth and to be life-long in its effects. It is characterised by difficulties with phonological processing, rapid naming, working memory, processing speed, and the automatic development of skills that may not match up to an individual's other cognitive abilities. It tends to be resistant to conventional teaching methods, but its effect can be mitigated by appropriately specific intervention, including the application of information technology and supportive counseling.
The following are some indicators of dyslexia:
A family history of dyslexia/reading difficulties
Persistent difficulties in getting dressed
Difficulties in clapping a simple rhyme
Difficulties with catching a ball, kicking or throwing a ball