sine qua non
/ˈsaɪnɪkweɪˈnɒn/noun
- an essential condition; a thing that is absolutely necessary必要条件; 必不可少的事物：
grammar and usage are the sine qua non of language teaching and learning.
语法和用法是语言教学中必不可少的内容。
词源
Latin， literally '(cause) without which not'.