Atomic energy is an outdated phrase which can mean a number of things related to energy produced by atoms:

• In the late-19th century through the early-20th century, it was often used to describe the particles ejected by radioactive elements (especially radium).
• From 1945 through the 1950s, it was often referred to energy released by nuclear reactions (especially nuclear fission and nuclear fusion), and in particular referred to either nuclear weapons or nuclear reactors.

A similar term used by the late-20th century, nuclear energy, is almost always used to mean the energy produced by nuclear reactors.