Nonproliferation is when something is kept from growing or spreading. Trying to reduce the number of nuclear weapons in the world is known as nuclear nonproliferation.
You're almost certain to find the noun nonproliferation in discussions about weapons of mass destruction, especially nuclear bombs and missiles. A nonproliferation treaty is an agreement between powerful countries to limit the number of nuclear weapons they have, with the eventual goal of disarmament, or completely eliminating them. The word has been around since 1965, from non-, "not," and proliferation, "increase or enlargement," from the French prolifère, "producing offspring."