the act or utterance of one who warns or the existence, appearance, sound, etc., of a thing that warns. something that serves to warn, give notice, or caution: We fired a warning at the intruders.

What kind of word is warning?

warning used as a noun:

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."

Where does the word warning come from?

warn (v.) Old English warnian "to give notice of impending danger," also intransitive, "to take heed," from Proto-Germanic *warōnan (source also of Old Norse varna "to admonish," Old High German warnon "to take heed," German warnen "to warn"), from PIE root *wer- (4) "to cover." Related: Warned; warning.