(set phrase) A friendly warning of something undesirable.

plural of fair warning

plural of warning sign

(attributive) Denoting equipment and systems designed to give sufficient warning of an impending event.

(military) A warning, relatively far in advance, of a potential attack.

plural of strategic warning

A bell rung to warn people of a danger. / (figuratively) A problematic or alarming aspect.

(American football) In the AFL, a warning given with one minute to go before the end of a half, after which special rules apply, eg, stoppage of the game clock after very first down.

(American football) In the NFL, a timeout and warning given to the coaches with two minutes to go before the end of each half.

(US) A warning, surrounded by a black border, affixed to a prescription drug to indicate that it may cause serious or life-threatening effects.