The Collaborative International Dictionary of English v.0.48:

guideline \guideline\ n. hypen is changed from WN: adjust hier.
   a formal rule describing how a situation must be handled; --
   used as a direction to administrators from superiors.

   Syn: guidelines.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

guideline
    n 1: a light line that is used in lettering to help align the
         letters
    2: a detailed plan or explanation to guide you in setting
       standards or determining a course of action; "the president
       said he had a road map for normalizing relations with
       Vietnam" [syn: road map, guideline]
    3: a rule or principle that provides guidance to appropriate
       behavior [syn: guidepost, guideline, rule of thumb]

Moby Thesaurus II by Grady Ward, 1.0:

96 Moby Thesaurus words for "guideline":
   Procrustean law, approach, arrangement, attack, blueprint,
   blueprinting, calculation, canon, charting, code, commandment,
   conception, contrivance, convention, criterion, design, device,
   dictum, disposition, enterprise, envisagement, figuring, foresight,
   forethought, form, formality, formula, formulary, game,
   general principle, golden rule, graphing, ground plan, guidelines,
   guiding principle, idea, imperative, intention, law, law of nature,
   layout, lineup, long-range plan, mapping, master plan, maxim,
   method, methodology, mitzvah, moral, norm, norma,
   operations research, order of nature, ordinance, organization,
   plan, planning, planning function, prearrangement, prescribed form,
   prescription, principium, principle, procedure, program,
   program of action, rationalization, regulation, rubric, rule,
   schedule, schema, schematism, schematization, scheme,
   scheme of arrangement, set form, settled principle, setup,
   standard, standing order, strategic plan, strategy, system,
   systematization, tactical plan, tactics, tenet, the big picture,
   the picture, universal law, way, working plan, working principle,
   working rule