register

A register is a variety of a language used for a particular purpose or in a particular social setting.

For example, an English speaker may adhere more closely to prescribed grammar, pronounce words ending in -ing with a velar nasal instead of an alveolar nasal (e.g. "walking", not "walkin'"), choose more formal words (e.g. train vs. choo-choo, sodium chloride vs. salt, child vs. kid, etc.), and refrain from using the word ain't when speaking in a formal setting, but the same person could violate all of these prescriptions in an informal setting.