The World Health Organisation (WHO) has this very good definition of
dyslexia in two of it's publications,

ICD-10, The International Statistical Classification of Diseases and
Related Health Problems, tenth revision
ICIDH-2, The International Classification of Impairments, Activities,
and Participation
These define dyslexia as:-

"A disorder manifested by difficulty learning to read, despite
conventional instruction, adequate intelligence and sociocultural
opportunity. It is dependent upon fundamental cognitive
disabilities which are frequently of constitutional origin".

This, I feel, is a very good definition as it says that dyslexia is
dependent on cognitive disabilities which are present from birth.
That also rules out such factors as poor teaching, social problems,
poor motivation as well as low intelligence.

If that dosn't make it a medical condition I don't know what would.
For more info goto http://www.who.int/en/ and use the search box to
search for dyslexia, icd-10 or ICIDH-2.

Harry