parchment vs vellum

parchment and vellum both are nouns.

Word	Noun	Adjective	Verb	Adverb
parchment	Yes	No	No	No
vellum	Yes	No	No	No

As nouns, vellum is a hyponym of parchment; that is, vellum is a word with a more specific, narrower meaning than parchment:

parchment: skin of a sheep or goat prepared for writing on
vellum: fine parchment prepared from the skin of a young animal e.g. a calf or lamb

parchment (noun)	vellum (noun)
skin of a sheep or goat prepared for writing on	fine parchment prepared from the skin of a young animal e.g. a calf or lamb
a superior paper resembling sheepskin	a heavy creamy-colored paper resembling parchment