What is sine qua non?

A sine qua non is a Latin phrase meaning "without which not." It refers to something that is essential or indispensable for a particular result or condition to occur.

In other words, it is a necessary condition without which something cannot happen. Originally a legal term, it is often used to describe the critical factor or element that must be present for an event or outcome.

For example, in law, if an action did not occur, a certain harm would not have happened; that action is the sine qua non of the harm. The phrase highlights the importance of an essential prerequisite in various contexts.