Definition
A self-sustaining sequence of events in which each occurrence triggers the next. In nuclear physics, it refers to a reaction in which neutrons released by the splitting (fission) of one atom strike other atoms, causing them to split and release more neutrons, which in turn split still more atoms — releasing large amounts of energy in the process.
Plain English
One event causes the next, which causes the next, and so on, with the process continuing on its own once it starts.
Context Anchor
Seen in discussions of combustion, fires, and other processes where one event can keep producing the conditions for the next event.
Derivation
Chain comes from the Latin catena, meaning a series of connected links. The image is of links pulling each other along — each one doing the same thing as the one before it, in sequence.
Why Pilots Care
Pilots care because some unsafe conditions can grow quickly once started. Understanding a chain reaction helps explain why stopping one required part of a fire or abnormal process can stop the whole sequence.
Analogy
Like a row of dominoes: knock one over and it knocks the next, which knocks the next, with no further help needed once the first one falls.
Intuition Check
Do not read “chain reaction” as just any series of events. In this technical sense, each reaction helps produce the next reaction.
Example Sentence 1
A nuclear reactor produces power by controlling the chain reaction of uranium atoms splitting and releasing energy.
Example Sentence 2
Mechanics check for conditions that could start a chain reaction before it leads to engine failure.