harshness vs stiffness

harshness and stiffness both are nouns.

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

As nouns, harshness and stiffness are synonyms defined as:

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

harshness (noun)	stiffness (noun)
the quality of being harsh or rough or grating to the senses	the physical property of being inflexible and hard to bend
excessive sternness	the property of moving with pain or difficulty
the roughness of a substance that causes abrasions	the inelegance of someone stiff and unrelaxed (as by embarrassment)
the quality of being cruel and causing tension or annoyance	excessive sternness
	firm resoluteness in purpose or opinion or action