[sahy-nee kwey non, kwah, sin-ey; Lat. si-ne kwah-nohn] 
| an indispensable condition, element, or factor; something essential: Her presence was the sine qua non of every social event. |
| si·ne qua non (sĭn'ĭ kwä nŏn', nōn', sī'nĭ, kwā) n. An essential element or condition: "The perfect cake is the sine qua non of the carefully planned modern wedding" (J.M. Hilary). [Late Latin sine quā (causā) nōn, without which (cause) not : Latin sine, without + Latin quā : ablative of quī, which, what, who + Latin nōn, not.] |
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.