plain vs flood plain

plain and flood plain both are nouns.

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

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

plain: extensive tract of level open land
flood plain: 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, floodplain, llano, moor, moorland, peneplain, peneplane, snowfield, steppe, tundra.

plain (noun)	flood plain (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)	flood plain (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)	flood plain (verb)
express complaints, discontent, displeasure, or unhappiness

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