The government issued a warning about unsafe drinking water. The shelf collapsed without any warning.
An example of a warning is a dog barking and growling in the middle of the night.
A word of warning to runners about the hot weather.
The incident is a warning to careless investors.
A warning light; warning words.
The boss gave him a warning that he would be fired if he did not desist from his behaviour.
An example of warning is a tornado siren; a warning siren.