/ ˈfɪʃn; NAmE / noun [ U ]
1.
(also ˌnuclear ˈfission ) ( physics ) the act or process of splitting the nucleus (= central part) of an atom, when a large amount of energy is released
—compare fusion
2.
( biology ) the division of cells into new cells as a method of reproducing cells
3.
( chemistry ) the breaking of a chemical bond between two atoms
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WORD ORIGIN
early 17th cent.: from Latin fissio(n-) , from findere to split.