Harm means to hurt or damage someone or something. When you harm someone, you cause them physical pain, emotional distress, or some kind of injury. A bully who pushes a classmate down the stairs causes physical harm. Someone who spreads cruel rumors causes emotional harm.

Harm can be intentional or accidental. You might harm a friendship by forgetting someone's birthday, even though you didn't mean to hurt their feelings. A factory might harm the environment by polluting a river, whether the owners intended to or not.

The word works as both a verb and a noun. You can harm someone (verb), or you can cause harm to someone (noun). When doctors promise to “do no harm,” they're pledging not to make patients worse while trying to help them.

Harmful describes something that causes harm: harmful chemicals, harmful habits, harmful advice. Harmless, on the other hand, means something won't hurt anyone. A harmless prank might surprise someone without actually upsetting them.