Actinide element

1. One of the group of chemical elements of increasing atomic number, starting with actinium (atomic number 89) and extending through atomic number 103. These elements occupy one single place in the extended periodic table, in the same group into which the rare-earth elements (lanthanides) are classified. See also: actinide
2. One of the radioactive elements, atomic numbers 89 to 103. Hurlbut
   Source: Dictionary of Mining, Mineral, and Related Terms