The multiplication factor, , is defined as (see Nuclear chain reaction): 
* If  is greater than 1, the chain reaction is supercritical, and the neutron population will grow exponentially. 
* If  is less than 1, the chain reaction is subcritical, and the neutron population will exponentially decay. 
* If , the chain reaction is critical and the neutron population will remain constant.
In an infinite medium, neutrons cannot leak out of the system and the multiplication factor becomes the infinite multiplication factor, , which is approximated by the four-factor formula.