any of the elements in the series () of radioactive, metallic chemical elements from element 89 (actinium) through element 103 (lawrencium): actinides resemble lanthanides in electronic structure
Origin:
< actin(ium) + -ide
See actinide in American Heritage Dictionary 4
noun
Any of a series of chemically similar, radioactive elements with atomic numbers ranging from 89 (actinium) through 103 (lawrencium).