The situation where one fission reaction instigates exactly one more reaction is called criticality.

Here's why:

* Criticality means that the chain reaction is self-sustaining. Each fission event produces exactly enough neutrons to cause one more fission.

* Subcriticality occurs when fewer than one new fission event is triggered per reaction, causing the chain reaction to die out.

* Supercriticality occurs when more than one new fission event is triggered per reaction, causing the chain reaction to grow exponentially.

Let me know if you'd like to know more about nuclear fission or criticality!