To make (someone) aware of (something impending); especially:

To caution or admonish (someone) against unwise or unacceptable behaviour.

“Don't let me catch you running in the corridor again, I warn you.”

(chiefly with "off", "away", and similar words) To advise or order to go or stay away.

“A sign warns trespassers off/away from the site.”

The action of the verb warn; an instance of warning someone.

Something spoken or written that is intended to warn.

“The boss gave him a warning that he would be fired if he did not desist from his behaviour.”

Used to warn of danger in signs and notices.