plainness vs pellucidity

plainness and pellucidity both are nouns.

Word	Noun	Adjective	Verb	Adverb
plainness	Yes	No	No	No
pellucidity	Yes	No	No	No

As nouns, pellucidity is a hypernym of plainness; that is, pellucidity is a word with a broader meaning than plainness:

plainness: clarity as a consequence of being perspicuous
pellucidity: free from obscurity and easy to understand; the comprehensibility of clear expression

Other hypernyms of plainness include clarity, clearness, limpidity, lucidity, lucidness.

plainness (noun)	pellucidity (noun)
the appearance of being plain and unpretentious	passing light without diffusion or distortion
an appearance that is not attractive or beautiful	free from obscurity and easy to understand; the comprehensibility of clear expression
clarity as a consequence of being perspicuous
the state of being unmixed with other material