public address system vs PA system

public address system and PA system both are nouns.

Word	Noun	Adjective	Verb	Adverb
public address system	Yes	No	No	No
PA system	Yes	No	No	No

As nouns, public address system and PA system are synonyms defined as:

public address system and PA system: an electronic amplification system used as a communication system in public areas

Other synonyms of public address system include P.A., P.A. system, PA.

public address system (noun)	PA system (noun)
an electronic amplification system used as a communication system in public areas	an electronic amplification system used as a communication system in public areas