linguistics vs descriptive

linguistics is a noun but descriptive is not a noun.

Word	Noun	Adjective	Verb	Adverb
linguistics	Yes	No	No	No
descriptive	No	Yes	No	No

linguistics (noun)	descriptive (noun)
the scientific study of language
the humanistic study of language and literature

linguistics (adjective)	descriptive (adjective)
	of or relating to an approach to linguistic analysis that aims at the description of a language's forms, structures and usage
	serving to describe or inform or characterized by description