pepper grinder vs pepper mill

pepper grinder and pepper mill both are nouns.

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

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

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