pepper mill vs pepper grinder

pepper mill and pepper grinder both are nouns.

Word	Noun	Adjective	Verb	Adverb
pepper mill	Yes	No	No	No
pepper grinder	Yes	No	No	No

As nouns, pepper mill and pepper grinder are synonyms defined as:

pepper mill (noun)	pepper grinder (noun)
a mill for grinding pepper	a mill for grinding pepper