The Collaborative International Dictionary of English v.0.48:

Prescriptive \Pre*scrip"tive\, a. [L. praescriptivus of a
   demurrer or legal exception.]
   1. (Law) Consisting in, or acquired by, immemorial or
      long-continued use and enjoyment; as, a prescriptive right
      of title; pleading the continuance and authority of long
      custom.
      [1913 Webster]

            The right to be drowsy in protracted toil has become
            prescriptive.                         --J. M. Mason.
      [1913 Webster]

   2. Of or pertaining to the doctrine that acceptable
      grammatical rules should be prescribed by authority,
      rather than be determined by common usage.
      [PJC]

WordNet (r) 3.0 (2006):

prescriptive
    adj 1: pertaining to giving directives or rules; "prescriptive
           grammar is concerned with norms of or rules for correct
           usage" [syn: prescriptive, normative] [ant:
           descriptive]

Moby Thesaurus II by Grady Ward, 1.0:

131 Moby Thesaurus words for "prescriptive":
   absolute, accepted, accustomed, acknowledged, admitted,
   authoritarian, authoritative, autocratic, average, binding,
   canonical, commanding, common, commonplace, compelling, compulsory,
   conclusive, conformable, constrictive, consuetudinary,
   conventional, current, customary, decisive, decretal, decretive,
   decretory, dictated, dictating, dictatorial, didactic, directive,
   dogmatic, entailed, established, ethnocentric, everyday,
   exceptional, excluding, exclusive, exclusory, familiar, final,
   fixed, folk, formulary, generally accepted, habitual, hallowed,
   handed down, hard and fast, hard-and-fast, heroic, hoary,
   household, immemorial, imperative, imperious, imposed,
   inadmissible, instructive, insular, inveterate, irrevocable,
   jussive, legendary, long-established, long-standing, mandated,
   mandating, mandatory, must, mythological, narrow, normal,
   normative, obligating, obligatory, obtaining, of long standing,
   of the folk, official, oral, ordinary, overbearing, parochial,
   peremptory, popular, preceptive, preclusive, predominating,
   prescribed, prescript, prevailing, prevalent, preventive,
   prohibitive, received, recognized, regular, regulation, required,
   restrictive, rooted, rubric, seclusive, segregative, select,
   selective, separative, set, snobbish, standard, statutory, stock,
   time-honored, traditional, tried and true, true-blue, ultimate,
   understood, universal, unwritten, usual, venerable, vernacular,
   widespread, without appeal, wonted, worshipful, xenophobic