Sine qua non or conditio sine qua non is a Latin legal term for "without which it could not be" ("but for"). It refers to an indispensable action, condition or thing.

Example

Mr. Wilson left his car but kept the key in the ignition. Dennis, 6-years-old and a.k.a. The Menace, started the car and backed it over Margaret.

Without Mr. Wilson's careless act, Dennis would not be driving the car. Therefore, Mr. Wilson's act was the conditio sine qua non of Margaret's injury.