To warn means to tell someone about a possible danger or problem before it happens, giving them a chance to avoid it or prepare for it. When dark clouds gather and thunder rumbles, that's nature warning you that a storm is coming. When a teacher warns the class that tomorrow's quiz will be challenging, she's helping students know they should study tonight.

A warning can be urgent and immediate: “Watch out!” you might shout to warn a friend who's about to step in a puddle. Or it can be calmer and more distant: a doctor might warn a patient that a certain medication could cause drowsiness.

The point of a warning is to help someone make better choices. A parent warns a child that the stove is hot. A friend warns you that a movie might be too scary. A sign warns drivers about a sharp curve ahead. Sometimes people ignore warnings and face consequences they could have avoided.

You can also warn someone against doing something, which means advising them not to do it: “I warned you against eating all that candy before dinner.” When someone refuses to listen to good advice, you might say, “Don't say I didn't warn you,” after the predictable result happens.