letterpress vs relief printing

letterpress and relief printing both are nouns.

Word	Noun	Adjective	Verb	Adverb
letterpress	Yes	No	No	No
relief printing	Yes	No	No	No

As nouns, letterpress and relief printing are synonyms defined as:

letterpress and relief printing: printing from a plate with raised characters

letterpress (noun)	relief printing (noun)
printing from a plate with raised characters	printing from a plate with raised characters