rigorousness vs severity

rigorousness and severity both are nouns.

Word	Noun	Adjective	Verb	Adverb
rigorousness	Yes	No	No	No
severity	Yes	No	No	No

As nouns, rigorousness and severity are synonyms defined as:

Other synonyms of rigorousness include asperity, grimness, hardship, rigor, rigour, rigourousness, severeness.

As nouns, rigorousness and severity are synonyms defined as:

Other synonyms of rigorousness include hardness, harshness, inclemency, rigor, rigour, rigourousness, severeness, stiffness.

rigorousness (noun)	severity (noun)
something hard to endure	excessive sternness
excessive sternness	used of the degree of something undesirable e.g. pain or weather
	something hard to endure
	extreme plainness