Abbreviation: ML = "Meniscus Protective Lens" (Nikon)
The abbreviation ML means "Meniscus Protective Lens" and is seen on Nikon products.
Explanation from Nikon
A curved meniscus protective glass element is installed in front of the lens to minimize ghosting by diffusing light re-reflected from the image sensor or film and the protective glass. Using a meniscus protective lens ensures a clear image with little ghosting.