market square vs open-air marketplace

market square and open-air marketplace both are nouns.

Word	Noun	Adjective	Verb	Adverb
market square	Yes	No	No	No
open-air marketplace	Yes	No	No	No

As nouns, market square and open-air marketplace are synonyms defined as:

market square and open-air marketplace: a public marketplace where food and merchandise is sold

Other synonyms of market square include open-air market.

market square (noun)	open-air marketplace (noun)
a public marketplace where food and merchandise is sold	a public marketplace where food and merchandise is sold