Material that contains unstable (radioactive) atoms that give off [ionizing]
radiation as they decay.
Glossary of Radiological Terms
Caesium - Countdown 2010 - Iodine - Radiation - Radioactivity - Strontium-90
To read about this term in context:
GreenFacts Summary on the Chernobyl Nuclear Accident:
Español: Material Radiactivo
Français: Substance radioactive