What is the X in X-mas?
1841, "division of a cell or organism," from Latin fissionem (nominative fissio) "a breaking up, cleaving," from past participle stem of findere "to split" (see fissure). Cognate with Old English bitan "to bite." Nuclear physics sense is 1939. As a verb, from 1929.
fission fis·sion (fĭsh'ən)
The act or process of splitting into parts.
The amitotic division of a cell or its nucleus.
An asexual process of reproduction in which a unicellular organism divides into two or more independently maturing daughter cells.
A nuclear reaction in which an atomic nucleus, especially a heavy nucleus such as an isotope of uranium, splits into fragments, usually two of comparable mass, with the evolution of from 100 million to several hundred million electron volts of energy.