The smallest mass of a fissionable material that will sustain a nuclear chain reaction.
The amount of matter needed to generate sufficient gravitational force to halt the current expansion of the universe. See also big bang, big crunch.
In physics, the amount of material that must be present before a chain reaction can sustain itself.
Note: The term critical mass is used to refer generally to the minimum amount of something needed to produce a given effect: “The town needs a critical mass of industry to attract more business.”