I Was Confused About The Term "Vellum"

It appears that "vellum" refers to a kind of parchment -- refers to it in English, that is. I had assumed, wrongly, that "vellum" was a Latin term for "parchment," alongside the more commonly-used "membranis." But no. "Vellum" may well be derived from the Latin "vellus," via French, but in Latin itself, as far as I am able to determine, having consulted many sources, there is no noun whose root form is "vellum" -- I refer to the root form because Latin nouns take on a variety of endings according to inflection -- and "vellus," apparently the Latin ancestor of the English "vellum," refers to "wool" or "fleece" or "hide" or "pelt," but not specifically to "parchment," although parchment is made from hides.