Heavy ion refers to an ionized atom which is usually heavier than carbon. The term is applied in reference to beams of particles used to produce heavy ion collisions in high energy nuclear physics (an area of research that is sometimes mistakenly included within particle physics or high energy physics). In nuclear physics, the atoms used as beam particles are generally completely ionized, so that they are bare atomic nuclei. 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, tungsten, gold, lead, and uranium.