calm vs placidity

calm and placidity both are nouns.

Word	Noun	Adjective	Verb	Adverb
calm	Yes	Yes	Yes	No
placidity	Yes	No	No	No

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

calm (noun)	placidity (noun)
steadiness of mind under stress	a disposition free from stress or emotion
wind moving at less than 1 knot; 0 on the Beaufort scale	a feeling of calmness; a quiet and undisturbed feeling

calm (adjective)	placidity (adjective)
(of weather) free from storm or wind
not agitated; without losing self-possession

calm (verb)	placidity (verb)
cause to be calm or quiet as by administering a sedative to
make steady
make calm or still
become quiet or calm, especially after a state of agitation