stiffness vs rigor

stiffness and rigor both are nouns.

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

As nouns, stiffness and rigor are synonyms defined as:

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

stiffness (noun)	rigor (noun)
the physical property of being inflexible and hard to bend	something hard to endure
the property of moving with pain or difficulty	the quality of being valid and rigorous
the inelegance of someone stiff and unrelaxed (as by embarrassment)	excessive sternness
excessive sternness
firm resoluteness in purpose or opinion or action