The policy or belief that a country should act on its own — deciding and doing things without waiting for allies, treaties, or international agreement. The -ism ending turns the one-off action into a whole way of thinking, so this is not a single decision but a habit or a stated position. You meet it almost only in politics, foreign policy, and academic writing: "a turn toward unilateralism", "critics accused the government of unilateralism". Its opposite is multilateralism, the belief in working through alliances and organisations like the UN. Almost every time you see unilateralism in print, it is an accusation. Supporters of the same policy would call it independence, sovereignty, or putting our country first. So the word tells you as much about the speaker as about the policy. One special case worth knowing: in British politics of the 1980s, unilateralism meant something narrower — the belief that Britain should give up its nuclear weapons alone, without waiting for other countries to do the same. If you read older British texts, that is likely the meaning.

單邊主義（外交上一國自行其是、不與盟友協商的立場）