pepper mill vs grinder

pepper mill and grinder both are nouns.

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

As nouns, grinder is a hypernym of pepper mill; that is, grinder is a word with a broader meaning than pepper mill:

Other hypernyms of pepper mill include mill, milling machinery.

pepper mill (noun)	grinder (noun)
a mill for grinding pepper	a machine tool that polishes metal
	machinery that processes materials by grinding or crushing
	grinding tooth with a broad crown; located behind the premolars
	a large sandwich made of a long crusty roll split lengthwise and filled with meats and cheese (and tomato and onion and lettuce and condiments); different names are used in different sections of the United States