The Collaborative International Dictionary of English v.0.48:

Regulation \Reg`u*la"tion\ (-l?"sh?n), n.
   1. The act of regulating, or the state of being regulated.
      [1913 Webster]

            The temper and regulation of our own minds.
                                                  --Macaulay.
      [1913 Webster]

   2. A rule or order prescribed for management or government;
      prescription; a regulating principle; a governing
      direction; precept; law; as, the regulations of a society
      or a school.
      [1913 Webster]

   Regulation sword, cap, uniform, etc. (Mil.), a sword,
      cap, uniform, etc., of the kind or quality prescribed by
      the official regulations.
      [1913 Webster]

   Syn: Law; rule; method; principle; order; precept. See
        Law.
        [1913 Webster]

WordNet (r) 3.0 (2006):

regulation
    adj 1: prescribed by or according to regulation; "regulation
           army equipment"
    n 1: an authoritative rule [syn: regulation, ordinance]
    2: a principle or condition that customarily governs behavior;
       "it was his rule to take a walk before breakfast"; "short
       haircuts were the regulation" [syn: rule, regulation]
    3: the state of being controlled or governed
    4: (embryology) the ability of an early embryo to continue
       normal development after its structure has been somehow
       damaged or altered
    5: the act of bringing to uniformity; making regular [syn:
       regulation, regularization, regularisation]
    6: the act of controlling or directing according to rule;
       "fiscal regulations are in the hands of politicians" [syn:
       regulation, regulating]

Moby Thesaurus II by Grady Ward, 1.0:

191 Moby Thesaurus words for "regulation":
   Procrustean law, accepted, accommodation, accustomed, act,
   adaptation, adjustment, administration, assimilation, attunement,
   authoritative, authority, average, balance, balancing, bill,
   binding, bylaw, canon, canonical, charting, civil government,
   coaptation, code, codification, command, commandment, common,
   commonplace, conduct, conformable, consuetudinary, control,
   convention, conventional, coordination, criterion, current,
   customary, decree, decretum, dictate, dictated, dictation, dictum,
   didactic, direction, directive, discipline, dispensation,
   disposition, edict, empery, empire, enactment, established,
   everyday, familiar, fiat, fixing, form, form of government,
   formality, formula, formulary, general orders, general principle,
   generally accepted, golden rule, governance, government, guidance,
   guideline, guiding principle, habitual, handling, hard and fast,
   harmonization, household, husbandry, imperative, institution,
   instruction, instructive, integration, jus, law, law of nature,
   lead, leading, legislation, lex, maintenance, management, managery,
   managing, mandatory, manipulation, maxim, measure, methodization,
   mitzvah, modification, modulation, moral, norm, norma, normal,
   normalization, normative, obtaining, official, order,
   order of nature, ordering, ordinance, ordinary, ordination,
   ordonnance, organization, oversight, pilotage, planning,
   political organization, polity, popular, precept, preceptive,
   predominating, prescribed, prescribed form, prescript,
   prescription, prescriptive, prevailing, prevalent, principium,
   principle, proclamation, pronouncement, rationalization, received,
   reconcilement, reconciliation, regime, regimen, regnancy, regular,
   regularization, reign, required, routinization, rubric, rule,
   ruling, running, set, set form, setting, settled principle,
   sovereignty, squaring, standard, standing order, statute,
   statutory, steerage, steering, stock, supervision, sway,
   synchronization, system of government, systematization, tenet,
   the conn, the helm, the wheel, time-honored, timing, traditional,
   typical, ukase, universal, universal law, usual, vernacular,
   widespread, wonted, working principle, working rule