A radiological weapon is any weapon that is designed to spread radioactivity, either to kill, or to deny the use of an area (a modern version of salting the earth) and consists of an device (such as a nuclear or conventional explosive) which spreads radioactive material. They have recently been called "dirty bombs", although that term more correctly refers to a type of nuclear weapon.
Radiological weapons are widely considered to be militarily useless for a state-sponsored army and are not believed to have been deployed by any military forces. Firstly, the use of such a weapon is of no use to an occupying force, as the target area becomes uninhabitable. Furthermore, area-denial weapons are generally of limited use to an attacking army as it slows the rate of advance so the need for a radioactive denial system is limited. Finally, like biological weapons, radiological weapons can take days to act on the opposing force. They therefore not only fail in neutralizing the opposing force instantly, but they also allow time for massive retaliation.
Means of radiological warfare that do not rely on any specific weapon, but rather on spreading radiation poisoning via a food chain or water table, seem to be more effective in some ways, but to share problems with chemical warfare.
Iraq under Saddam Hussein is reported to have tested a radiological weapon in 1987 for use against Iran. This weapon was found to be impractical because the radioactive isotopes in the weapon would decay quickly, rendering it useless within a week after the weapon was manufactured. Furthermore, it was found that for the radioactive material to spread, weather conditions had to be ideal. These problems are in general shared by all forms of air-borne radiological warfare.
Useless as they may be to an ordinary military force, the weapons have been suggested as a possible terror weapon in order to create panic in densely populated areas. They do not require weapons-grade materials, and common materials such as Cesium-137, used in radiological medical equipment, could be used. In fact even very mild sources would likely be enough to cause panic. Anything from dynamite to compressed air could be used to create an aerosol of the material, or it could be dumped from the air using crop dusters - the latter use however being not a weapon so much as a means of warfare involving many different components.