critical mass
noun [ C usually singular or U ] uk/ˌkrɪt.ɪ.kəl ˈmæs/ us/ˌkrɪt̬.ɪ.kəl ˈmæs/
physics specialized临界质量（实现核子连锁反应所需要的物质的最小量）
the smallest amount of matter that is needed to produce a nuclear chain reaction
产生质变所需的量
the size that something needs to reach before a particular change, event, or development can happen