The minimum quantity of fissile material required for a controlled chained nuclear reaction is called Critical Mass. It depends on the properties of nuclear material used. It also depends on the shape, temperature, density, purity and enrichment of the nuclear material. Critical Mass is utilized while designing the nuclear weapons.
Diagrams
Originated from/Coined by
Lise Meitner, Otto Hahn, Otto Frisch
Further Reading/ References
https://en.wikipedia.org/wiki/Critical_mass