initialism vs abbreviation

initialism and abbreviation both are nouns.

Word	Noun	Adjective	Verb	Adverb
initialism	Yes	No	No	No
abbreviation	Yes	No	No	No

As nouns, abbreviation is a hypernym of initialism; that is, abbreviation is a word with a broader meaning than initialism:

initialism: an abbreviation formed from the initial letters of the several words in the name and pronounced separately
abbreviation: a shortened form of a word or phrase

initialism (noun)	abbreviation (noun)
an abbreviation formed from the initial letters of the several words in the name and pronounced separately	shortening something by omitting parts of it
a word formed from the initial letters of the several words in the name and pronounced as one word	a shortened form of a word or phrase