When one fission reaction instigates exactly one more fission reaction it is called?

Answer: controller.

Justification:

A fission reaction is the split of the nucleus of an atom due to the bombardment with neutrons.

A fission reaction may terminate in a reaction chain, a controlled reaction or extinguish.

When for every 2 or 3 neutrons released, only one neutron is allowed to strike a fissionable nucleus, the reaction is controlled.

When the the ratio is less than one then the reaction will be extinguished.

When the number of reactions grow, this is a reaction chain, where the reaction is out of control, and the consequence is the release of vast amount of energy (heat) is a short time, causing a nuclear explosion.

It is a controller chain reaction