When an object gives off a certain kind of energy, like the sun or an x-ray machine, it can be described as radioactive.
The adjective radioactive is a scientific term for a particular type of energy-emitting substance or thing. Radioactive energy, or radiation, is made up of active particles or electromagnetic waves. Nuclear waste is radioactive, but so are more ordinary things, like cell phones and the food we eat. The French word for it is radio-actif, which was invented by Pierre and Marie Curie, who combined radiationem, Latin for "a shining" with actif, French for "active."