Heavy ion refers to an ionized atom which is usually heavier than helium. Heavy-ion physics is devoted to the study of extremely hot nuclear matter and the collective effects appearing in such systems, differing from particle physics, which studies the interactions between elementary particles. Together, these two subjects are denoted high-energy physics. In nuclear physics, the nuclei used as beam particles are generally completely ionized. The nuclei can be directed to a fixed target, or can be split into two beams moving in opposite directions that are brought into collision at a well-defined spot.

Heavy ion nuclei most often used in nuclear physics experiments include carbon, silicon, copper, tungsten, gold, lead, and uranium.