fissure vs slit

fissure and slit both are nouns.

Word	Noun	Adjective	Verb	Adverb
fissure	Yes	No	Yes	No
slit	Yes	No	Yes	No

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

fissure (noun)	slit (noun)
(anatomy) a long narrow slit or groove that divides an organ into lobes	a long narrow opening
a long narrow cleft	obscene terms for female genitals
a long narrow depression in a surface	a narrow fissure
	a depression scratched or carved into a surface

fissure (verb)	slit (verb)
break into fissures or fine cracks	cut a slit into
	make a clean cut through