first floor vs ground level

first floor and ground level both are nouns.

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

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

first floor and ground level: 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 floor.

first floor (noun)	ground level (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
	the height of the ground on which something stands