The process in which the nucleus of an atom splits into two lighter atoms, releasing a large amount of energy, is .

Explanation:

A process in which a large nuclei of an atom splits into two small nuclei is known as a nuclear fission.

For example, is a nuclear fission reaction.

And, when two small nuclei combine together and result in the formation of a large nuclei then it is known as nuclear fusion.

For example, is a nuclear fusion reaction.

Thus, we can conclude that the process in which the nucleus of an atom splits into two lighter atoms, releasing a large amount of energy, is nuclear fission.