severity vs stiffness

severity and stiffness both are nouns.

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

As nouns, severity and stiffness are synonyms defined as:

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

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