The Collaborative International Dictionary of English v.0.48:

authoritarian \authoritarian\ adj.
   1. characteristic of an absolute ruler or absolute rule;
      having absolute sovereignty; -- of governments or rulers;
      as, an authoritarian regime

   Syn: autocratic, dictatorial, despotic, tyrannical
        [WordNet 1.5]

   2. 1 expecting unquestioning obedience: "he was imperious and
      dictatorial"; "the timid child of authoritarian parents";
      "insufferably overbearing behavior toward the waiter"

   Syn: dictatorial, overbearing
        [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

authoritarian \authoritarian\ n.
   1. 1 behaves like a tyrant.

   Syn: dictator
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

authoritarian
    adj 1: characteristic of an absolute ruler or absolute rule;
           having absolute sovereignty; "an authoritarian regime";
           "autocratic government"; "despotic rulers"; "a
           dictatorial rule that lasted for the duration of the
           war"; "a tyrannical government" [syn: authoritarian,
           autocratic, dictatorial, despotic, tyrannic,
           tyrannical]
    2: expecting unquestioning obedience; "the timid child of
       authoritarian parents"; "insufferably overbearing behavior
       toward the waiter" [syn: authoritarian, dictatorial,
       overbearing]
    n 1: a person who behaves in a tyrannical manner; "my boss is a
         dictator who makes everyone work overtime" [syn:
         authoritarian, dictator]

Moby Thesaurus II by Grady Ward, 1.0:

158 Moby Thesaurus words for "authoritarian":
   Spartan, Spartanic, absolute, absolutist, absolutistic, arbitrary,
   aristocratic, arrogant, ascendant, astringent, austere,
   authoritative, authorized, autocratic, autonomous, bigot, bigoted,
   borne, bossy, bureaucratic, civic, civil, closed,
   clothed with authority, commanding, competent, consequential,
   considerable, constitutional, constricted, controlling, cramped,
   creedbound, deaf, deaf to reason, demanding, democratic, despotic,
   dictatorial, doctrinaire, dogmatic, dominant, domineering, dour,
   duly constituted, eminent, empowered, ex officio, exacting,
   exigent, fanatical, fascist, fascistic, federal, federalist,
   federalistic, feudal, governing, governmental, great, grim,
   grinding, gubernatorial, harsh, heavy-handed, hegemonic,
   hegemonistic, heteronomous, hidebound, high-handed, illiberal,
   imperative, imperial, imperious, important, influential, insular,
   leading, little, little-minded, lordly, magisterial, magistral,
   masterful, matriarchal, matriarchic, mean, mean-minded,
   mean-spirited, meticulous, mighty, momentous, monarchal,
   monarchial, monarchic, monocratic, narrow, narrow-hearted,
   narrow-minded, narrow-souled, narrow-spirited, nearsighted,
   official, oligarchal, oligarchic, oppressive, overbearing,
   overruling, parliamentarian, parliamentary, parochial, patriarchal,
   patriarchic, peremptory, petty, pluralistic, political, potent,
   powerful, preeminent, prestigious, prominent, provincial, puissant,
   purblind, ranking, repressive, republican, rugged, ruling,
   self-governing, senior, severe, shortsighted, small, small-minded,
   stern, straitlaced, strict, stringent, stuffy, substantial,
   superior, suppressive, supreme, theocratic, total, totalitarian,
   tough, tyrannical, tyrannous, uncatholic, uncharitable, ungenerous,
   unliberal, unsparing, unyielding, weighty