severity vs rigor

severity and rigor both are nouns.

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

As nouns, severity and rigor are synonyms defined as:

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

As nouns, severity and rigor are synonyms defined as:

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

severity (noun)	rigor (noun)
excessive sternness	something hard to endure
used of the degree of something undesirable e.g. pain or weather	the quality of being valid and rigorous
something hard to endure	excessive sternness
extreme plainness