Define fission and give an example of a reaction

It's a process in which a heavy nucleus is broken down into two or more lighter fragments ...

it is used in nuclear reactors and atom bombs

the opposite i. e when two or more lighter nuclei joins to give a heavy one is called Fusion, it is used in Hydrogen bombs

Definition: The action of dividing or splitting something into two or more parts.

Reaction Example: Autotomy, sometimes termed transverse fission, is the name given to a process of unequal fission in which a portion of the body separates off with subsequent regeneration.