Great Britain vs pillar box

Great Britain and pillar box both are nouns.

Word	Noun	Adjective	Verb	Adverb
Great Britain	Yes	No	No	No
pillar box	Yes	No	No	No

Great Britain (noun)	pillar box (noun)
an island comprising England and Scotland and Wales	a red pillar-shaped letter box
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