pass away vs stifle

pass away is not a noun while stifle is a noun.

Word	Noun	Adjective	Verb	Adverb
pass away	No	No	Yes	No
stifle	Yes	No	Yes	No

As verbs, stifle is a hyponym of pass away; that is, stifle is a word with a more specific, narrower meaning than pass away:

pass away: pass from physical life and lose all bodily attributes and functions necessary to sustain life
stifle: be asphyxiated; die from lack of oxygen

Other hyponyms of pass away include abort, asphyxiate, suffocate, buy it, pip out, drown, predecease, famish, starve, fall, succumb, yield.

pass away (noun)	stifle (noun)
	joint between the femur and tibia in a quadruped; corresponds to the human knee

pass away (verb)	stifle (verb)
pass from physical life and lose all bodily attributes and functions necessary to sustain life	suppress or constrain so as to lessen in intensity
go out of existence	be asphyxiated; die from lack of oxygen
	suppress in order to conceal or hide
	impair the respiration of or obstruct the air passage of