When the Curies saw radium glowing in a test tube at the turn of the last century, they knew that they were seeing something new for which no word existed. The word they coined, radioactive, was a combination of the Latin word radius meaning a ray and the French word actif from the Latin word actus meaning a doing, a driving, impulse; a part in a play. The Curies were attempting to convey what they were seeing-energy radiating out from the material. Today, radioactive decay is defined as the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles. There are many types of radioactivity classified today, but all fall under the larger definition of activity radiating out in rays.