sine qua non
si·ne qua non / ˌsini ˌkwä ˈnōn; ˌsini ˌkwä ˈnän/ • n. an essential condition; a thing that is absolutely necessary: grammar and usage are the sine qua non of language teaching and learning.
Sine Qua Non
SINE QUA NON
[Latin, Without which not.] A description of a requisite or condition that is indispensable.
In the law of torts, a causal connection exists between a particular act and an injury when the injury would not have arisen but for the act. This is known as the but for rule or sine qua non rule.