public square vs plaza

public square and plaza both are nouns.

Word	Noun	Adjective	Verb	Adverb
public square	Yes	No	No	No
plaza	Yes	No	No	No

As nouns, plaza is a hyponym of public square; that is, plaza is a word with a more specific, narrower meaning than public square:

Other hyponyms of public square include piazza, place.

public square (noun)	plaza (noun)
a place of assembly for the people in ancient Greece	mercantile establishment consisting of a carefully landscaped complex of shops representing leading merchandisers; usually includes restaurants and a convenient parking area; a modern version of the traditional marketplace
an open area at the meeting of two or more streets	a public square with room for pedestrians