stiffness vs severity

stiffness and severity both are nouns.

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

As nouns, stiffness and severity are synonyms defined as:

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

stiffness (noun)	severity (noun)
the physical property of being inflexible and hard to bend	excessive sternness
the property of moving with pain or difficulty	used of the degree of something undesirable e.g. pain or weather
the inelegance of someone stiff and unrelaxed (as by embarrassment)	something hard to endure
excessive sternness	extreme plainness
firm resoluteness in purpose or opinion or action