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To split up.

In physics, fission is a process where a large nucleus such as 235U is caused to split into two smaller nuclei. The process is initiated by the capture of a slow (often called thermal) neutron.

235U + n --> 236U*

(The * on the Uranium-236 indicates that it is in a highly excited state)

The new nucleus then splits into two daughter nuclei. This split can happen in a number of ways but one example might be:

236U* --> 140Xe+ 94Sr + 2n

This process releases roughly 200MeV per event, plus two more neutrons. Unfortunately these neutrons are way too energetic to be captured by other nuclei, so in nuclear reactors or nuclear bombs they are slowed down by a moderator in order to produce a chain reaction

In biology, fission refers to the process whereby a cell divides. There are two types of cellular fission: mitosis and meiosis.

Perhaps these entries should be refactored into nuclear fission and cellular fission? Similarly with fusion, since cells can fuse to form hybridomas, or a virus and a cell undergo membrane fusion, etc.

If you do that, links to this page will probably not work 
since people rarely use the longer phrase. Personally I like
the page as it is.