Another common meaning for the word which Webster does not cover (due to the fact that nuclear physics wasn't as big a field in 1913 as it is now) is the process where an atomic nucleus splits into two or more lighter nuclei, releasing radiation, fast neutrons and large amounts of energy in the process. Today, the word 'fission' generally refers to this meaning in popular conversation, due to the importance of nuclear energy to all of our lives (although nuclear fission would be a better term - as Webster notes, the unadorned term 'fission' has other meanings).

Unstable radioisotope nuclei will fission spontaneously, and certain other nuclei not normally considered 'unstable' (such as 235U) may be made to fission with the application of slow neutrons. These slow neutrons are usually created in a chain reaction, since they are otherwise quite expensive to generate. The neutrons have to be slow because 'fast' ones aren't as readily captured by atomic nuclei, and are thus 'wasted'. Since most neutrons resulting from fission are 'fast', they must be slowed down by a moderator such as heavy water or graphite to cause further fissionings.

Side note: Only nuclei heavier than iron nuclei will yield energy when fissioned, and only nuclei much larger than iron nuclei will yield economic amounts of energy when they do so. Fissioning an element that is lighter than iron actually consumes energy. The converse is also true - fusing elements lighter than iron yields energy, fusing elements heavier than iron consumes it. Iron is therefore the ash of nuclear fires of all kinds. Supernovae are caused when a massive star runs out of lighter elements to fuse than iron - since it cannot sustain a fusion reaction anymore, it is caused to explode. Due to the large amounts of energy present in the explosion, some iron and other nuclei are made to fuse, creating heavier elements and 'charging' the nuclear 'batteries' of heavy elements. We are all made of stardust!