si·ne qua non 
(sĭn′ĭ kwä nŏn, nōn, sī′nĭ, kwā)
(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 : feminine ablative of quī, which + Latin nōn, not.]