stiffness vs sternness

stiffness and sternness both are nouns.

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

As nouns, sternness is a hypernym of stiffness; that is, sternness is a word with a broader meaning than stiffness:

Other hypernyms of stiffness include strictness.

stiffness (noun)	sternness (noun)
the physical property of being inflexible and hard to bend	uncompromising resolution
the property of moving with pain or difficulty	the quality (as of scenery) being grim and gloomy and forbidding
the inelegance of someone stiff and unrelaxed (as by embarrassment)
excessive sternness
firm resoluteness in purpose or opinion or action