- the minimum amount of fissile material that can sustain a nuclear chain reaction under a given set of conditions
- the minimum amount or number required for something to happen, begin, etc.
critical masscritical mass
- The smallest mass of a fissionable material that will sustain a nuclear chain reaction at a constant level.
- The total mass of matter needed to generate sufficient gravitational force to halt the current expansion of the universe.
- An amount or level needed for a specific result or new action to occur: “[Plastic bags are] so nearly weightless that it's difficult to gather enough of a critical mass to make recycling them economically worthwhile” ( Susan Freinkel )