The term dirty bomb was first used to refer to any nuclear weapon that generated a significant amount of radioactive waste in the form of nuclear fallout. Due to the inefficiency of early nuclear weapons (as low as 2% or less), they tended to disperse large amounts of unused fissile material. The term does not necessarily refer to a specific nuclear weapon design, but is typically used to contrast with newer, more efficient bombs. Some nuclear weapon designs feature the inclusion of a salting metal which will create large amounts of long-lasting fallout radiation (most commonly cobalt) when radiated by the weapon core.
Another usage of the term is to talk about radiological weapons. It refers especially to a weapon which would disperse radioactive material through conventional explosives. The term was put in focus on June 10, 2002, when U.S. officials announced they had captured an al-Qaida terrorist named Josť Padilla in Chicago's O'Hare International Airport a month earlier who was allegedly planning for such a device.