A chain reaction, in general, is any self-sustaining molecular or nuclear reaction, the products of which contribute to the propagation of the reaction. In particular a fission chain reaction is a process in which one nuclear transformation is capable of initiating a chain of similar transformations.
For example, when nuclear fission occurs in a uranium-235 nucleus, between 2 and 3 neutrons are emitted, each of which is capable of causing the fission of further uranium-235 nuclei. The chain reaction so created is the basis of the atomic bomb and the nuclear reactor. If the average number of transformations directly caused by one transformation is less than one, the reaction is said to be convergent or sub-critical; if it is equal to one, the reaction is self-sustained or critical; if it exceeds one, the reaction is divergent or super-critical.