The essential, crucial, or indispensable ingredient without which something would be impossible: “Her leadership was the sine qua non of the organization's success.” From Latin, meaning “without which nothing.”
sine qua non
An essential element or condition, as in A perfect cake is the since qua non of a birthday party. This phrase is Latin for "without which not" and has been used in English since about 1600. It appears more in writing than in speech.