The Collaborative International Dictionary of English v.0.48:

Preparatory \Pre*par"a*to*ry\, a. [L. praeparatorius: cf. F.
   pr['e]paratoire.]
   Preparing the way for anything by previous measures of
   adaptation; antecedent and adapted to what follows;
   introductory; preparative; as, a preparatory school; a
   preparatory condition.
   [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "preparatory":
   antecedent, anterior, basic, before, chief, elementary, essential,
   exordial, first, foremost, fundamental, headmost,
   in preparation for, inaugural, inductive, initiatory, introductory,
   leading, opening, precedent, preceding, precessional, precursory,
   prefatory, prelim, preliminary, preludial, prelusive, preparative,
   prerequisite, prevenient, primary, prior, proemial, propaedeutic,
   provident, provisional, rudimentary