What is the medical term for a developmental reading disorder is known as what?

Dyslexia is the term meaning reading disorder.
dyslexia
I think the disorder you are looking for is:

Dyslexia. which effects 5% - 17% of the US Population alone.

Now, not all "disorders" are bad, there is also

Hyperlexia, which is a sort of positive disorder, which usually shows that a child has a high IQ, as well as a High reading level for their ages.
I think dyslexia is the term to which you are referring.