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]