noun
1. : a series of events so related to each other that each one initiates the succeeding one especially in such a manner as to produce a cumulative effect
the employee with more retention points can and does bump the employee with fewer points, setting off a costly chain reaction down the line — Sam Stavisky
2. : a chemical reaction (as polymerization) that once started can maintain itself by interaction of the starting materials with transitory reactive products (as atoms or free radicals), more of the transitory products being formed as they are consumed
3. : a self-propagating fission of atomic nuclei continued by the further action of one of the products (as in the fission of a uranium nucleus by a neutron that causes the release of more neutrons that cause further fissions and so on)