Ain't

Wikipedia article
Examples of linguistic prescription

For example, a descriptive linguist (descriptivist) working in English would describe the word "ain't" in terms of usage, distribution, and history rather than correctness; while acknowledging it a nonstandard form, the descriptivist would accept the broad principle that as a language evolves it often incorporates such items and thus would not didactically reject the term as never appropriate. A prescriptivist, on the other hand, would rule on whether "ain't" met some criterion of rationality, historical grammatical usage, or conformity to a contemporary standard dialect. Frequently this standard dialect is associated with the upper class (e.g., Great Britain's Received Pronunciation). When a form does not conform — as is the case for "ain't" — the prescriptivist will condemn it as a solecism or barbarism or common in the sense of vulgar, prescribing that it not be used. In short, the door is absolutely barred to nonstandard forms.