Stolen straight from Wikipedia:
Myopia, which is measured in diopters by the strength or optical power of a corrective lens that focuses distant images on the retina, has also been classified by degree or severity:
Low myopia usually describes myopia of −3.00 diopters or less (i.e. closer to 0.00).
Medium myopia usually describes myopia between −3.00 and −6.00 diopters.
High myopia usually describes myopia of −6.00 or more.
Source, slightly changed: http://en.wikipedia.org/wiki/Myopia
