paperknife vs letter opener

paperknife and letter opener both are nouns.

Word	Noun	Adjective	Verb	Adverb
paperknife	Yes	No	No	No
letter opener	Yes	No	No	No

As nouns, paperknife and letter opener are synonyms defined as:

paperknife and letter opener: dull knife used to cut open the envelopes in which letters are mailed or to slit uncut pages of books

Other synonyms of paperknife include paper knife.

paperknife (noun)	letter opener (noun)
dull knife used to cut open the envelopes in which letters are mailed or to slit uncut pages of books	dull knife used to cut open the envelopes in which letters are mailed or to slit uncut pages of books