fetus vs abortus

fetus and abortus both are nouns.

Word	Noun	Adjective	Verb	Adverb
fetus	Yes	No	No	No
abortus	Yes	No	No	No

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

fetus: an unborn or unhatched vertebrate in the later stages of development showing the main recognizable features of the mature animal
abortus: a human fetus whose weight is less than 0.5 kilogram when removed or expelled from the mother's body

Other hyponyms of fetus include monster, teras, baby.

fetus (noun)	abortus (noun)
an unborn or unhatched vertebrate in the later stages of development showing the main recognizable features of the mature animal	a human fetus whose weight is less than 0.5 kilogram when removed or expelled from the mother's body