Question

Define fission and give an example of a reaction

Answer 1

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.

Answer 2

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