In nuclear engineering, critical mass refers to the smallest amount of fissile material needed for a sustained nuclear chain reaction/06%3A_Nuclear_Weapons-Fission_and_Fusion/6.04%3A_The_Manhattan_Project-_Critical_Mass_and_Bomb_Construction). The critical mass of a fissionable material depends on its nuclear properties, density, shape, enrichment, and purity. A numerical measure of critical mass is dependent on the effective neutron multiplication factor k, which is the average number of neutrons released per fission event that go on to cause another fission event rather than being absorbed or leaving the material. When k = 1, the mass is critical, and the chain reaction is self-sustaining. The amount of a fissionable material that will support a self-sustaining chain reaction is a critical mass/06%3A_Nuclear_Weapons-Fission_and_Fusion/6.04%3A_The_Manhattan_Project-_Critical_Mass_and_Bomb_Construction). The critical mass of a fissionable substance is the minimum amount of fissionable material that will support a self-sustaining chain reaction.

The critical mass of a fissionable material depends on several factors, including the shape of the material, its composition and density, and the level of purity. A sphere has the minimum possible surface area for a given mass, and hence minimizes the leakage of neutrons. By surrounding the fissionable material with a suitable neutron "reflector," the loss of neutrons can be reduced, and the critical mass can be reduced.

In a broader sense, critical mass can also refer to a size, number, or amount large enough to produce a particular result. For example, the critical mass of activity needed for a retail store refers to the minimum amount of activity required to produce a particular result.

It is important to note that the term "critical mass" has different meanings depending on the context. In the context of nuclear engineering, it refers specifically to the minimum amount of fissile material needed for a sustained nuclear chain reaction.