The Collaborative International Dictionary of English v.0.48:

Prerequisite \Pre*req"ui*site\, a.
   Previously required; necessary as a preliminary to any
   proposed effect or end; as, prerequisite conditions of
   success.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Prerequisite \Pre*req"ui*site\, n.
   Something previously required, or necessary to an end or
   effect proposed.
   [1913 Webster]

         The necessary prerequisites of freedom.  -- Goldsmith.
   [1913 Webster]

WordNet (r) 3.0 (2006):

prerequisite
    adj 1: required as a prior condition or course of study
    n 1: something that is required in advance; "Latin was a
         prerequisite for admission" [syn: prerequisite,
         requirement]

Moby Thesaurus II by Grady Ward, 1.0:

108 Moby Thesaurus words for "prerequisite":
   arrangement, bare necessities, basic training, boundary condition,
   briefing, call for, called for, catch, clause, clearing the decks,
   condition, demand, demand for, demanded, desideration, desideratum,
   donnee, equipment, escalator clause, escape clause, escape hatch,
   essential, essentials, familiarization, fine print, fixing,
   foundation, given, grounds, groundwork, imperative, indicated,
   indispensable, irreducible, irreductible, irreplaceable, joker,
   kicker, limiting condition, makeready, making ready, manufacture,
   mobilization, must, must item, necessaries, necessary, necessities,
   necessitous, necessity, need, need for, needed, needful,
   obligation, obligatory, occasion, parameter, planning,
   prearrangement, precondition, preliminaries, preliminary,
   preliminary act, preliminary step, prep, preparation, preparative,
   preparatory, preparatory study, preparing, prepping,
   prerequirement, pretreatment, processing, propaedeutic, provident,
   provision, provisional, provisions, proviso, qualification,
   readying, required, requirement, requisite, requisition,
   saving clause, sine qua non, small print, spadework, specification,
   stipulation, string, terms, the necessary, the needful, training,
   treatment, trial, tryout, ultimatum, unforgoable, vital, want,
   wanted, warm-up, whereas