The term Dyslexia is in widespread usage.  
It is one of several Specific Learning Difficulties (SpLD).

Dyslexia is derived from two Greek words, 

         “dys”  – meaning difficulty and “lexia” meaning words.  

The literal meaning is therefore difficulty with words, or difficulty reading and spelling words.

The Rose Report on Dyslexia (2009) has defined Dyslexia as follows (page 10):  

“Dyslexia is a learning difficulty that primarily affects the skills involved in accurate and fluent word reading and spelling.”

Also the Rose Report comments that characteristic features are: 

• Difficulties in phonological awareness, verbal memory and verbal processing speed 
• Occurrence across the range of intellectual abilities 
• Of a continuum, not a distinct category, and there are no clear cut-off points.  
• Co-occurring of difficulties that may be seen in aspects of language, motor co-ordination, mental calculation, concentration and personal organisation (though these are not, by themselves, markers of dyslexia) 

Note that a good indication of the severity and persistence of dyslexic difficulties can be gained by examining how the individual responds or has responded to well-founded intervention.  

We also accept and use the following definitions:  

DfES and British Dyslexia Association   

“Dyslexia is a learning difference, a combination of strengths and weaknesses, which affect the learning process in reading, spelling and writing and sometimes numeracy. Dyslexic learners may also have accompanying weaknesses in short-term memory, sequencing and the speed at which they process information.”                                                                                   DfES & BDA (2005) p.23 now DCSF   

British Psychological Society  

“Dyslexia is evident when accurate and fluent word reading and/or spelling develops very incompletely or with great difficulty. This focuses on literacy learning at the word level and implies that the problem is severe and persistent despite appropriate learning opportunities. It provides the basis for a staged process of assessment through teaching.”    Working Party of the DECP, 1999, (2005 reprint) p.11   

These definitions, taken together, explain that dyslexia is a recognisable PATTERN of strengths and difficulties that PERSISTS over time. The pattern can look different in different people.