first floor vs ground floor

first floor and ground floor both are nouns.

Word	Noun	Adjective	Verb	Adverb
first floor	Yes	No	No	No
ground floor	Yes	No	No	No

As nouns, first floor and ground floor are synonyms defined as:

first floor and ground floor: the floor of a building that is at or nearest to the level of the ground around the building

Other synonyms of first floor include ground level.

first floor (noun)	ground floor (noun)
the floor of a building that is at or nearest to the level of the ground around the building	the floor of a building that is at or nearest to the level of the ground around the building
	becoming part of a venture at the beginning (regarded as position of advantage)