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warn‧ing1 S3 W2

1 [uncountable and countable] something, especially a statement, that tells you that something bad, dangerous, or annoying might happen so that you can be ready or avoid it COLLOCATIONS 
warning that without (any) warning issue a warning give/sound a warning heed a warning (=take notice of it) advance/prior warning (=when you tell someone about something so they know it will happen) health warning (=a warning that something is bad for your health) a word of warning (=used before a warning when giving advice)

warning that without (any) warning issue a warning give/sound a warning heed a warning (=take notice of it) advance/prior warning (=when you tell someone about something so they know it will happen) health warning (=a warning that something is bad for your health) a word of warning (=used before a warning when giving advice)
2 [countable] a statement telling someone that if they continue to behave in an unsatisfactory way, they will be punished :

a warning of floods