The Collaborative International Dictionary of English v.0.48:

Prescribe \Pre*scribe"\, v. t. [imp. & p. p. Prescribed; p. pr
   & vb. n. Prescribing.] [L. praescribere, praescriptum; prae
   before + scriebe to write. See Scribe.]
   [1913 Webster]
   1. To lay down authoritatively as a guide, direction, or rule
      of action; to impose as a peremptory order; to dictate; to
      appoint; to direct.
      [1913 Webster]

            Prescribe not us our duties.          --Shak.
      [1913 Webster]

            Let streams prescribe their fountains where to run.
                                                  --Dryden.
      [1913 Webster]

   2. (Med.) To direct, as a remedy to be used by a patient; as,
      the doctor prescribed quinine.
      [1913 Webster]

   Syn: To appoint; order; command; dictate; ordain; institute;
        establish.
        [1913 Webster]

WordNet (r) 3.0 (2006):

prescribed
    adj 1: set down as a rule or guide
    2: fixed or established especially by order or command; "at the
       time appointed (or the appointed time") [syn: appointed,
       decreed, ordained, prescribed]
    3: conforming to set usage, procedure, or discipline; "in
       prescribed order" [syn: official, prescribed]
    4: formally laid down or imposed; "positive laws" [syn:
       positive, prescribed]

Moby Thesaurus II by Grady Ward, 1.0:

58 Moby Thesaurus words for "prescribed":
   accepted, authoritative, binding, bound, bounded, canonical,
   conditioned, confined, conformable, consuetudinary, conventional,
   copyrighted, cramped, current, customary, dictated, didactic,
   disciplined, established, everyday, familiar, finite, formulary,
   generally accepted, hard and fast, instructive, limited, mandatory,
   moderated, narrow, normal, obtaining, official, ordinary, patented,
   popular, preceptive, prescript, prescriptive, prevalent,
   proscribed, qualified, received, regular, regulation, restricted,
   rubric, set, standard, statutory, stock, strait, straitened,
   time-honored, traditional, usual, widespread, wonted