Bad means not good, harmful, or of poor quality. When milk goes bad, it spoils and shouldn't be drunk. A bad storm might damage houses and knock down trees. A bad grade on a test shows you didn't understand the material well enough.

The word covers everything from minor problems to serious ones. A bad haircut is disappointing but fixable. A bad decision, like texting while riding a bike, could lead to real danger. When someone feels bad, they might be sick, sad, or guilty about something they did wrong.

Bad can describe actions, outcomes, or character. A person who consistently makes bad choices or treats others poorly might be called bad, though most people do both good and bad things. Weather, food, ideas, and situations can all be bad in different ways.

The opposite of bad is good. Sometimes people use bad casually (“That movie was so bad!”) and sometimes seriously (“Cheating is bad.”) Context tells you how serious the problem is. When something goes from bad to worse, it means a difficult situation is getting even harder to handle.

Interestingly, in some slang contexts, people ironically use bad to mean impressively good, as in “That skateboard trick was bad!” But in standard usage, bad means exactly what it says: something undesirable, incorrect, or harmful.