Sine qua non or conditio sine qua non was originally a Latin legal term for "without which it could not be" ("but for"). It refers to an indispensable and essential action, condition, or ingredient. In recent times it has passed from a merely legal usage to a more general usage in many languages, including English, French, Italian, etc.
Example
Mr. Wilson left his car but kept the key in the ignition. Dennis, 6-years-old, 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.
The content of this page is retrieved from http://en.wikipedia.org/wiki/Sine_qua_non under GFDL
