flood plain vs champaign

flood plain and champaign both are nouns.

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

As nouns, champaign is a hypernym of flood plain; that is, champaign is a word with a broader meaning than flood plain:

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

Other hypernyms of flood plain include field, plain.

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