excess vs plethora

excess and plethora both are nouns.

Word	Noun	Adjective	Verb	Adverb
excess	Yes	Yes	No	No
plethora	Yes	No	No	No

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

excess: immoderation as a consequence of going beyond sufficient or permitted limits
plethora: extreme excess

excess (noun)	plethora (noun)
excessive indulgence	extreme excess
immoderation as a consequence of going beyond sufficient or permitted limits
a quantity much larger than is needed
the state of being more than full

excess (adjective)	plethora (adjective)
more than is needed, desired, or required