fusion - the process of forming new atomic nuclei by the fusion of lighter ones; principally the formation of helium nuclei by the fusion of hydrogen and its isotopes. The energy released in the process is referred to as nuclear energy or fusion energy. The controlled release of energy from the thermonuclear fusion reaction between deterium and tritium nuclei (requiring very high temperature and pressure) has been attained for short periods in experimental reactors and has been proposed as the basis for power generation, plasma - very high temeprature ionized gaseous discharge in which there is no resultant charge, the number of positive and negative ions being equal, in addition to unionized molecules or atoms