WordNet (r) 3.0 (2006):

sine qua non
    n 1: a prerequisite [syn: essential condition, sine qua non]

Moby Thesaurus II by Grady Ward, 1.0:

70 Moby Thesaurus words for "sine qua non":
   bench mark, boundary condition, cardinal point, catch, chief thing,
   clause, climax, condition, core, cornerstone, crisis,
   critical point, crux, donnee, escalator clause, escape clause,
   escape hatch, essence, essential, essential matter, fine print,
   fundamental, gist, given, gravamen, great point, grounds, heart,
   high point, important thing, issue, joker, kernel, keystone,
   kicker, landmark, limiting condition, main point, main thing,
   material point, meat, milestone, must, necessity, nub, obligation,
   parameter, pith, pivot, prerequisite, provision, provisions,
   proviso, real issue, requirement, requisite, salient point,
   saving clause, small print, specification, stipulation, string,
   substance, substantive point, terms, the bottom line, the point,
   turning point, ultimatum, whereas