An element member of the actinide group of 15 radioactive elements.
A group of 15 radioactive elements some of which occur naturally while others are produced in nuclear reactions. They include plutonium, americium and neptunium. The health hazard presented by the actinides, if they are released into the environment, comes from the potency of their radioactive characteristics. They are alpha-emitters, and therefore can cause intense localized damage in tissues if absorbed into the body.
Any of the 14 radioactive elements of the periodic table that are positioned under the lanthanides, to which they have similar chemistry.