severeness vs rigor

severeness and rigor both are nouns.

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

As nouns, severeness and rigor are synonyms defined as:

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

As nouns, severeness and rigor are synonyms defined as:

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

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