pass away vs predecease

pass away and predecease both are verbs.

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

As verbs, predecease is a hyponym of pass away; that is, predecease 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
predecease: die before; die earlier than

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

pass away (verb)	predecease (verb)
pass from physical life and lose all bodily attributes and functions necessary to sustain life	die before; die earlier than
go out of existence