The Collaborative International Dictionary of English v.0.48:

Authoritative \Au*thor"i*ta*tive\, a.
   1. Having, or proceeding from, due authority; entitled to
      obedience, credit, or acceptance; determinate; commanding.
      [1913 Webster]

            The sacred functions of authoritative teaching.
                                                  --Barrow.
      [1913 Webster]

   2. Having an air of authority; positive; dictatorial;
      peremptory; as, an authoritative tone.
      [1913 Webster]

            The mock authoritative manner of the one, and the
            insipid mirth of the other.           --Swift.
      [1913 Webster] -- Au*thor"i*ta*tive*ly, adv. --
      Au*thor"i*ta*tive*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

authoritative
    adj 1: having authority or ascendancy or influence; "an
           important official"; "the captain's authoritative manner"
           [syn: authoritative, important]
    2: of recognized authority or excellence; "the definitive work
       on Greece"; "classical methods of navigation" [syn:
       authoritative, classical, classic, definitive]
    3: sanctioned by established authority; "an authoritative
       communique"; "the authorized biography" [syn:
       authoritative, authorized, authorised]

Moby Thesaurus II by Grady Ward, 1.0:

265 Moby Thesaurus words for "authoritative":
   Christian, Daedalian, absolute, absolutist, absolutistic, accepted,
   accurate, acquainted with, adept, adroit, approved, apt, arbitrary,
   aristocratic, armipotent, arrogant, artistic, ascendant, assuring,
   at home in, at home with, attested, authentic, authenticated,
   authoritarian, authorized, autocratic, binding, bossy, bravura,
   brilliant, canonical, cathedral, certified, charismatic, charming,
   circumstantiated, clean, clever, clothed with authority, cogent,
   commanding, competent, conclusive, confined, confirmed,
   consequential, considerable, consistent, controlling, conventional,
   conversant with, convictional, convincing, coordinated, correct,
   crack, crackerjack, cunning, customary, cute, daedal, decisive,
   deft, dependable, despotic, determinative, dexterous, dextrous,
   dictated, dictatorial, didactic, diplomatic, doctrinaire,
   documented, dogmatic, dominant, domineering, duly constituted,
   dynamic, effective, effectual, efficacious, eminent, empowered,
   enchanting, energetic, estimable, evangelical, ex cathedra,
   ex officio, excellent, expert, factual, faithful, familiar with,
   fancy, feature, featured, feudal, firm, forceful, forcible,
   formulary, good, goodish, governing, graceful, great, grinding,
   handy, hard and fast, hegemonic, hegemonistic, high-handed,
   high-potency, high-powered, high-pressure, high-tension,
   imperative, imperial, imperious, important, impressive, in force,
   in power, indisputable, influential, informed in, ingenious,
   instructive, intimate with, irrefutable, irresistible, just,
   knowledgeable, lawful, leading, learned, legal, legitimate,
   limited, literal, logical, lordly, magisterial, magistral,
   magnetic, mandatory, master of, masterful, masterly, mighty,
   mighty in battle, momentous, monocratic, neat, no mean,
   of the faith, official, operative, oppressive, orthodox,
   orthodoxical, overbearing, overruling, peremptory, personable,
   persuasive, politic, potent, powerful, preceptive, preeminent,
   prepotent, prescribed, prescript, prescriptive, prestigious,
   professional, proficient, proficient in, prominent, proper, proven,
   puissant, quick, quite some, ranking, ready, received, regulation,
   reliable, repressive, reputable, resourceful, restricted, right,
   rubric, ruling, sanctioned, satisfactory, satisfying, scholarly,
   scriptural, self-consistent, senior, severe, skillful, slick,
   solid, some, sound, specialist, specialistic, specialized,
   standard, statesmanlike, statutory, strict, striking, strong,
   strong in, stylish, suasive, substantial, sufficient, superior,
   suppressive, supreme, sure, tactful, technical, telling, textual,
   the compleat, the complete, totalitarian, traditional,
   traditionalistic, true, true-blue, trustable, trustworthy,
   truthful, tyrannical, tyrannous, unrefutable, up on, valid,
   validated, verifiable, verified, veritable, versed in, vigorous,
   virtuoso, vital, weighty, well-done, well-founded, well-grounded,
   well-read in, winning, workmanlike