sine qua non (SY-nee kway NON) noun
An indispensable condition; prerequisite. [Latin sine qua non (without which not).]
"Of course, the mastery of language is not the sine qua non of competence in literacy. But ease in communication in the language shows how one's cognitive abilities have developed."
-- Kenendy Buhere; Proposal to Split TSC Ill-thought; The Kenya Times (Nairobi); Jan 19, 2007.