champaign vs floodplain

champaign and floodplain both are nouns.

Word	Noun	Adjective	Verb	Adverb
champaign	Yes	No	No	No
floodplain	Yes	No	No	No

As nouns, floodplain is a hyponym of champaign; that is, floodplain is a word with a more specific, narrower meaning than champaign:

champaign: extensive tract of level open land
floodplain: a low plain adjacent to a river that is formed chiefly of river sediment and is subject to flooding

Other hyponyms of champaign include flat, flood plain, llano, moor, moorland, peneplain, peneplane, snowfield, steppe, tundra.

champaign (noun)	floodplain (noun)
extensive tract of level open land	a low plain adjacent to a river that is formed chiefly of river sediment and is subject to flooding