The term that describes the reaction process in a fission reaction is nuclear fission.

Here's a breakdown:

* Nuclear: Refers to the nucleus of an atom, the central part containing protons and neutrons.

* Fission: Means "to split." In nuclear fission, a heavy atomic nucleus (like uranium) is split into two or more lighter nuclei, releasing a tremendous amount of energy.

So, nuclear fission encompasses the entire process of splitting a heavy nucleus and the subsequent release of energy.