1. The act or process of splitting into parts.

2. A nuclear reaction in which an atomic nucleus, especially a heavy nucleus, such as an isotope of uranium, splits into fragments, usually two fragments of comparable but unequal mass, and releases a few neutrons and about 100 million electron volts of energy. Nuclear fission may occur spontaneously or may be induced by the absorption of a neutron, which can initiate a nuclear chain reaction.

3. Biology An asexual reproductive process in which a unicellular organism divides into two or more independently maturing daughter cells.