What is meant by a " chain reaction "?

When the energy released in a reaction causes new reactions to occur, it is called a chain reaction. In a chain reaction, the reaction keeps on occurring in a sequence, producing huge amounts of energy.

a chemical reaction or other process in which the products themselves promote or spread the reaction, which under certain conditions may accelerate dramatically.

Explanation:

the self-sustaining fission reaction spread by neutrons which occurs in nuclear reactors and bombs.

a series of events, each caused by the previous one.