stiffen vs alter

stiffen and alter both are verbs.

Word	Noun	Adjective	Verb	Adverb
stiffen	No	No	Yes	No
alter	No	No	Yes	No

As verbs, alter is a hypernym of stiffen; that is, alter is a word with a broader meaning than stiffen:

Other hypernyms of stiffen include change, modify.

stiffen (verb)	alter (verb)
become stiff or stiffer	remove the ovaries of
make stiff or stiffer	become different in some particular way, without permanently losing one's or its former characteristics or essence
severely restrict in scope or extent	cause to change; make different; cause a transformation
	insert words into texts, often falsifying it thereby
	make an alteration to