first estate vs Great Britain

first estate and Great Britain both are nouns.

Word	Noun	Adjective	Verb	Adverb
first estate	Yes	No	No	No
Great Britain	Yes	No	No	No

first estate (noun)	Great Britain (noun)
the clergy in France and the heads of the church in Britain	an island comprising England and Scotland and Wales
	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