letterhead vs stationery

letterhead and stationery both are nouns.

Word	Noun	Adjective	Verb	Adverb
letterhead	Yes	No	No	No
stationery	Yes	No	No	No

As nouns, stationery is a hypernym of letterhead; that is, stationery is a word with a broader meaning than letterhead:

letterhead: a sheet of stationery with name and address of the organization printed at the top
stationery: paper cut to an appropriate size for writing letters; usually with matching envelopes

Other hypernyms of letterhead include letter paper.

letterhead (noun)	stationery (noun)
a sheet of stationery with name and address of the organization printed at the top	paper cut to an appropriate size for writing letters; usually with matching envelopes