mountain vs inundation

mountain and inundation both are nouns.

Word	Noun	Adjective	Verb	Adverb
mountain	Yes	No	No	No
inundation	Yes	No	No	No

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

Other hyponyms of mountain include deluge, flood, torrent, haymow.

mountain (noun)	inundation (noun)
a land mass that projects well above its surroundings; higher than a hill	the rising of a body of water and its overflowing onto normally dry land
(often followed by `of') a large number or amount or extent	an overwhelming number or amount