To retaliate is to do something bad to someone because they have already done something bad to you. You hit back. It can be a punch, a sharp reply, a week of not speaking, or — in the news — a country firing back or putting extra taxes on another country’s goods. The word does not say the first person was right or wrong; it only says the second action is payback. It is a bit more formal than get them back or get revenge, which is what people say at home. You retaliate against someone, or you retaliate by doing something. Though actually, people sometimes use it for any response that feels like a slap, even a joke that goes a bit far; the tidy idea is “harm in return,” but real talk is looser. It is not the same as defend, which is about stopping harm, not returning it.