floodplain vs flood plain

floodplain and flood plain both are nouns.

Word	Noun	Adjective	Verb	Adverb
floodplain	Yes	No	No	No
flood plain	Yes	No	No	No

As nouns, floodplain and flood plain are synonyms defined as:

floodplain and flood plain: a low plain adjacent to a river that is formed chiefly of river sediment and is subject to flooding

floodplain (noun)	flood plain (noun)
a low plain adjacent to a river that is formed chiefly of river sediment and is subject to flooding	a low plain adjacent to a river that is formed chiefly of river sediment and is subject to flooding