What term did Marie Curie coin to describe the emission of rays by uranium?

The term Marie Curie coined for the emission of rays by uranium was radioactivity. She derived the term from radioactif, the French word meaning radioactive, while investigating the unusual rays first observed from uranium compounds.

In 1896, Henri Becquerel discovered that uranium salts could darken a photographic plate even when protected from sunlight. Curie made the phenomenon the subject of her doctoral research and used sensitive electrical measurements to show that the effect was an intrinsic property of uranium atoms. She called this property radioactivity, helping establish that the radiation did not depend on chemical reactions or prior exposure to light.

Marie and Pierre Curie's work broadened the idea beyond uranium. They found that thorium was radioactive and identified two new elements, polonium and radium, in uranium ore. The word originally described radiation from certain heavy elements, but scientists later generalized it to the spontaneous emission of ionizing radiation from unstable atomic nuclei. “Radioactive decay” is related but different: it describes the nuclear process that produces the radiation, while radioactivity names the property or phenomenon.