Sea of Japan vs East Sea

Sea of Japan and East Sea both are nouns.

Word	Noun	Adjective	Verb	Adverb
Sea of Japan	Yes	No	No	No
East Sea	Yes	No	No	No

As nouns, Sea of Japan and East Sea are synonyms defined as:

Sea of Japan and East Sea: an arm of the Pacific bordered by Japan, Korea, North Korea, and Russia

Sea of Japan (noun)	East Sea (noun)
an arm of the Pacific bordered by Japan, Korea, North Korea, and Russia	an arm of the Pacific bordered by Japan, Korea, North Korea, and Russia