usage

As an adjective usage

is used.

As a noun usagist is

(linguistics|rare) one who defines a language by usage rather than theory.

As adjectives the difference between usage and descriptivist

is that usage is used while descriptivist is subscribing to, or related to, descriptivism.

As a noun descriptivist is

an advocate of descriptivism; one who believes a language is defined by usage rather than theory.