Final answer:
The term 'critical mass' refers to the minimum quantity of fissionable material required to sustain a nuclear fission chain reaction. It varies based on several factors including material type, purity, and shape.
Step-by-step explanation:
To have a self-sustaining nuclear fission reaction in a given material, a critical mass of fissionable material is required. This critical mass is the minimum amount of substance that can maintain a chain reaction, where the neutrons produced can sustain the process by inducing further fission events. The amount needed for critical mass varies depending on the fissionable material's type, purity, temperature, shape, and the way in which neutron reactions are moderated. If the mass of the material is above the critical mass, the system is considered supercritical, and if it is below, it is subcritical. Nuclear fission can thus either be controlled or uncontrolled based on the amount of material present, criticality, and the control mechanisms in place.