plain vs floodplain

plain and floodplain both are nouns.

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

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

plain: 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 plain include flat, flood plain, llano, moor, moorland, peneplain, peneplane, snowfield, steppe, tundra.

plain (noun)	floodplain (noun)
a basic knitting stitch made by putting the needle through the front of the stitch from the lefthand side	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

plain (adjective)	floodplain (adjective)
not elaborate or elaborated; simple
lacking patterns especially in color
free from any effort to soften to disguise
clearly revealed to the mind or the senses or judgment
not mixed with extraneous elements
lacking embellishment or ornamentation
lacking in physical beauty or proportion

plain (verb)	floodplain (verb)
express complaints, discontent, displeasure, or unhappiness

plain (adverb)	floodplain (adverb)
unmistakably (`plain' is often used informally for `plainly')