critical mass 
noun [in singular]
1 Physics The minimum amount of fissile material needed to maintain a nuclear chain reaction.Just two months earlier, the Observer published internal documents indicating that there had been eight breaches of safety rules intended to prevent critical masses of fissile material being brought together....
- Its basic principle is assembling a critical mass of a radioactive element with properties to spark a swift chain reaction.
- Such explosions are caused by the accumulation of a critical mass of material falling onto the neutron star from a partner.
2The minimum size or amount of resources required to start or maintain a venture: some of our new industries are now reaching critical mass...
- The multicultural marketplace has reached critical mass, and it requires a new approach.
- But until the market reaches a critical mass, dealers have to turn to other revenue streams to keep going.
- Stephens said the company is talking to a number of larger domain name registrars about using their services as a channel to reach the required critical mass of customers.