flood plain vs floodplain

flood plain and floodplain both are nouns.

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

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

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

flood plain (noun)	floodplain (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