si•ne qua non
Pronunciation: (sī'nē kwā non', kwä, sin'ā; Lat. si'ne kwä-nōn'), [key]
an indispensable condition, element, or factor; something essential: Her presence was the sine qua non of every social event.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.