- Radioactive: Atoms that are unstable. An element may have several forms, or isotopes. A radioactive isotope of an element may be called "radioactive". However, the correct term is radionuclide.
- Radiation: Emissions given off by way of mass, energy or both from unstable atoms or high-voltage devices.
- Decay: When an atom emits an alpha or beta particle or gamma ray, it becomes a different type of atom. Radioactive substances may go through several stages of decay before they change into a stable, or non-ionizing, form. For example: U-238 has 14 different stages of decay before it stabilizes.
- Half-life: Each radionuclide has a characteristic half-life, which is the time require for half of the quantity of material to decay.
- Background: Radiation the occurs naturally everywhere in our environment.
- Dose: A general term used to refer to the effect on a material that is exposed to radiation. It is used to refer either to the amount of energy absorbed by a material exposed to radiation (see Dose, absorbed) or to the potential biological effect in tissue exposed to radiation (see Dose, equivalent).
- Dose Rate: The rate at which radiation is being absorbed.