letter paper vs stationery

letter paper and stationery both are nouns.

Word	Noun	Adjective	Verb	Adverb
letter paper	Yes	No	No	No
stationery	Yes	No	No	No

As nouns, letter paper and stationery are synonyms defined as:

letter paper and stationery: paper cut to an appropriate size for writing letters; usually with matching envelopes

letter paper (noun)	stationery (noun)
paper cut to an appropriate size for writing letters; usually with matching envelopes	paper cut to an appropriate size for writing letters; usually with matching envelopes
writing paper for use in writing correspondence