antiballistic missile vs ABM

antiballistic missile and ABM both are nouns.

Word	Noun	Adjective	Verb	Adverb
antiballistic missile	Yes	No	No	No
ABM	Yes	No	No	No

As nouns, antiballistic missile and ABM are synonyms defined as:

antiballistic missile and ABM: a defensive missile designed to shoot down incoming intercontinental ballistic missiles

antiballistic missile (noun)	ABM (noun)
a defensive missile designed to shoot down incoming intercontinental ballistic missiles	a defensive missile designed to shoot down incoming intercontinental ballistic missiles