One of a series of chemically similar, radioactive elements with atomic numbers ranging from 89 (actinium) through 103 (lawrencium); actinide series: One kind of actinide is a neptunium, which is a hard, silvery, and ductile radioactive metal.

Some of the related chemical elements of the actinide series include actinium, thorium, palladium, uranium, neptunium, plutonium, americium, curium, berkelium, californium, einsteinium, fermium, mendelevium, nobelium, and lawrencium.