A
learning disability, technically defined as a minimum 20 point drop in apparent IQ as soon as
fine motor skills are required. It shows up most in
handwriting -- people with dysgraphia have
messy handwriting and often have to write slower to get the correct letters to come out.
Not everyone with messy handwriting is dysgraphic, though. In many cases, practice of the right sort will improve handwriting. Dysgraphic people are, in general, unable to improve. I was lucky; my third-grade teacher realized I would never write any better (and that, in fact, printing was better for me than cursive) and set me up with the school secretary to learn touch-typing. Later on, though, this disability actually cost me a level or two of letter grades until my parents decided to have me tested by a neuropsychologist. The tests ranged widely, including your basic ancient mental health questionaire, hand strength tests, and logic puzzles. Once this was done, I had dispensation to do things like use a typewriter on exams, which made my life much easier.
As might be guessed, this is related to dyslexia and dyscalcula, both in derivation and the actual problem in the brain itself.