omission vs ellipsis

omission and ellipsis both are nouns.

Word	Noun	Adjective	Verb	Adverb
omission	Yes	No	No	No
ellipsis	Yes	No	No	No

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

omission: any process whereby sounds or words are left out of spoken words or phrases
ellipsis: omission or suppression of parts of words or sentences

Other hyponyms of omission include aphaeresis, apheresis, aphesis, elision, eclipsis.

omission (noun)	ellipsis (noun)
a mistake resulting from neglect	omission or suppression of parts of words or sentences
neglecting to do something; leaving out or passing over something
any process whereby sounds or words are left out of spoken words or phrases
something that has been omitted