Harmful means causing or likely to cause damage, injury, or negative effects. When something is harmful, it hurts people, animals, the environment, or important relationships and institutions.

Harmful things range from obvious physical dangers to subtler problems. Touching a hot stove is harmful because it burns your skin. Eating spoiled food is harmful because it can make you sick. But harmful can also describe things that damage in less visible ways: spreading rumors is harmful to friendships, and constant criticism can be harmful to someone's confidence.

Scientists study whether chemicals, foods, or behaviors are harmful to human health. Doctors warn that too much sun exposure is harmful to skin. Environmentalists work to reduce practices that are harmful to oceans and forests.

The opposite of harmful is beneficial or helpful. Something neutral neither helps nor hurts. When making choices, it helps to think through what might be harmful and what might be beneficial. A little screen time might be fine, but too much can be harmful to your sleep and concentration. One piece of candy is a treat, but eating candy all day would be harmful to your teeth and overall health.

People often say “no harm done” when a mistake didn't actually cause any damage, or “more harm than good” when an action backfires and creates worse problems than it solves.