Great Britain vs mere

Great Britain and mere both are nouns.

Word	Noun	Adjective	Verb	Adverb
Great Britain	Yes	No	No	No
mere	Yes	Yes	No	No

Great Britain (noun)	mere (noun)
an island comprising England and Scotland and Wales	a small pond of standing water
a monarchy in northwestern Europe occupying most of the British Isles; divided into England and Scotland and Wales and Northern Ireland; `Great Britain' is often used loosely to refer to the United Kingdom

Great Britain (adjective)	mere (adjective)
	apart from anything else; without additions or modifications
	being nothing more than specified