Non-proliferation is the effort to stop certain weapons — above all nuclear bombs, and also chemical and biological weapons — from spreading to more countries. It is not only 'please make fewer bombs.' It is especially 'countries that do not have these weapons yet should not start, and countries that have them should not help others build them.' You will hear it in news, treaties, and UN talk. The famous agreement is the Nuclear Non-Proliferation Treaty. Though actually, the word sounds tidy, and the politics are not: countries argue about who already 'counts' as a nuclear power, who may run civilian nuclear plants, and whether the big powers are cutting their own weapons. Ordinary people rarely say this word; they say 'stopping the spread of nuclear weapons.' In speeches it is often set next to disarmament, which is a bigger hope: fewer weapons, or none, in the countries that already have them.