Fission, a splitting of something into two or more parts, may refer to: 1  Fission (biology), the subdivision of a cell or multi-cellular body into one or more parts and the regeneration of each part into a complete individual Mitochondrial fission. 2  Loop fission, a computer science concept. Nuclear fission, where a large atomic nucleus (such as that of uranium) is split into two (or sometimes more) smaller nuclei Cold fission.