Also called an Improvised Nuclear Explosive Device (INED).

Improvised nuclear devices or (INDs) are theoretical illicit nuclear weapons bought, stolen, or otherwise originating from a nuclear state, or a weapon fabricated by a terrorist group from illegally obtained fissile nuclear weapons material that produces a nuclear explosion. An IND would be built from the components of a stolen weapon or from scratch using nuclear material (plutonium or highly enriched uranium). It would produce same physical and medical effects as nuclear weapon. A successful detonation would result in catastrophic loss of life, destruction of infrastructure, and nuclear contamination of a very large area. (Wikipedia)

Department of Homeland Security definition:

An improvised nuclear device (IND) is a device built from components of a stolen weapon or from scratch using nuclear material that could produce nuclear explosions.

Related Links

• Wikipedia Entry on Improvised Nuclear Devices