Radiothorium is a radioactive element that has various synonyms. One of the common synonyms for radiothorium is thorium-X, which was originally used to describe the decay product of thorium. Another synonym is ionium, which is closely associated with thorium in terms of its radioactivity and chemical properties. Additionally, radiothorium is sometimes referred to as mesothorium, especially in older scientific literature. Finally, the term radiothorium is often used interchangeably with the term thorium-228, which refers to the most stable and abundant isotope of this element. These synonyms are useful for researchers and scientists who deal with the intricate details of nuclear chemistry and physics.