stiffness vs severeness

stiffness and severeness both are nouns.

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

As nouns, stiffness and severeness are synonyms defined as:

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

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