art vs plastic art

art and plastic art both are nouns.

Word	Noun	Adjective	Verb	Adverb
art	Yes	No	No	No
plastic art	Yes	No	No	No

As nouns, plastic art is a hyponym of art; that is, plastic art is a word with a more specific, narrower meaning than art:

art (noun)	plastic art (noun)
the creation of beautiful or significant things	the arts of shaping or modeling; carving and sculpture
the products of human creativity; works of art collectively
a superior skill that you can learn by study and practice and observation
photographs or other visual representations in a printed publication