emblem vs medallion

emblem and medallion both are nouns.

Word	Noun	Adjective	Verb	Adverb
emblem	Yes	No	No	No
medallion	Yes	No	No	No

As nouns, medallion is a hyponym of emblem; that is, medallion is a word with a more specific, narrower meaning than emblem:

emblem (noun)	medallion (noun)
special design or visual object representing a quality, type, group, etc.	an award for winning a championship or commemorating some other event
a visible symbol representing an abstract idea	an emblem indicating that a taxicab is registered
	a circular helping of food (especially a boneless cut of meat)
	a circular helping of food (especially a boneless cut of meat) "medallions of veal"
	any of various large ancient Greek coins