critical mass
noun
1- Physics the minimum amount of fissile material needed to maintain a nuclear chain reaction【物理】临界质量。
1.1
- figurative the minimum size or amount of resources required to start or maintain a venture〈喻〉（创办或维持风险企业所需的）最小规模（或资源数量）：
a communication system is of no value unless there is a critical mass of users.
除非有足够数量的用户, 否则通信系统就没有价值。