startle vs flinch

startle and flinch both are nouns.

Word	Noun	Adjective	Verb	Adverb
startle	Yes	No	Yes	No
flinch	Yes	No	Yes	No

As nouns, flinch is a hyponym of startle; that is, flinch is a word with a more specific, narrower meaning than startle:

Other hyponyms of startle include startle reaction, startle response, Moro reflex, startle reflex, wince.

startle (noun)	flinch (noun)
a sudden involuntary movement	a reflex response to sudden pain

startle (verb)	flinch (verb)
to stimulate to action	draw back, as with fear or pain
move or jump suddenly, as if in surprise or alarm