an element with atomic number of 89 (actinium) to 102. Usually applied to those above uranium - 93 up (also called transuranics). Actinides are radioactive and typically have long half-lives. They are therefore significant in wastes arising from nuclear fission, eg used fuel. They are fissionable in a fast reactor.
Elements 89-102 are called actinides. Electrons added during the Aufbau construction of actinide atoms go into the 5f subshell. Actinides are unstable and undergo radioactive decay. The most common actinides on Earth are uranium and thorium.
Are radioactive elements with an atomic number larger than 88.
The actinide series encompasses the 15 chemical elements that lie between actinium and lawrencium on the periodic table, with atomic numbers 89 - 103http://www.iupac.org/reports/provisional/abstract04/connelly_310804.html IUPAC Provisional Recommendations for the Nomenclature of Inorganic Chemistry (2004) (online draft of an updated version of the "Red Book" IR 3-6). The actinide series is named after actinium.