a statement or event that warns of something or that serves as a cautionary example

police issued a warning about fake £20 notes

his sad death should be a warning to everyone

a red warning light

inform someone in advance of a possible danger, problem, or other unpleasant situation

his father had warned him of what might happen

‘He's going to humiliate you,’ John warned

traffic signals warned of fog

the union warned that its members were going on strike