A nuclear deterrent is the phrase used to refer to a country's nuclear weapons arsenal, when considered in the context of deterrence theory.
Deterrence theory holds that nuclear weapons are intended to deter other states from attacking with their nuclear weapons, through the promise of retaliation and mutually assured destruction (MAD). It can also be assigned as a response to an attack by conventional forces; for example, the doctrine of massive retaliation threatened to launch US nuclear weapons in response to Soviet attacks.
In order for a nuclear deterrent to be successful, a country must preserve its second-strike capability. A nuclear deterrent is sometimes composed of a nuclear triad, as in the case of the nuclear weapons owned by the United States. Other countries, such as the United Kingdom, have only sea-based nuclear weapons.
