The Collaborative International Dictionary of English v.0.48:

Decorum \De*cor"um\, n. [L. dec[=o]rum, fr. dec[=o]rus. See
   Decorous.]
   Propriety of manner or conduct; grace arising from
   suitableness of speech and behavior to one's own character,
   or to the place and occasion; decency of conduct; seemliness;
   that which is seemly or suitable.
   [1913 Webster]

         Negligent of the duties and decorums of his station.
                                                  --Hallam.
   [1913 Webster]

         If your master
         Would have a queen his beggar, you must tell him,
         That majesty, to keep decorum, must
         No less beg than a kingdom.              --Shak.

   Syn: Decorum, Dignity.

   Usage: Decorum, in accordance with its etymology, is that
          which is becoming in outward act or appearance; as,
          the decorum of a public assembly. Dignity springs from
          an inward elevation of soul producing a corresponding
          effect on the manners; as, dignity of personal
          appearance.
          [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

79 Moby Thesaurus words for "decorum":
   amenities, appropriateness, becomingness, bienseance,
   ceremoniousness, civilities, civility, comity, conformity,
   convenance, convention, conventional usage, conventionalism,
   conventionality, correctitude, correctness, courtliness, custom,
   decencies, decency, decorousness, delicacy, deportment, dignity,
   diplomatic code, elegance, elegancies, etiquette,
   exquisite manners, felicity, fitness, fittingness, form,
   formalities, formality, genteelness, gentility, good form,
   good manners, goodness, happiness, mannerliness, manners, meetness,
   modesty, mores, natural politeness, niceness, normality,
   normativeness, order, orderliness, point of etiquette, politeness,
   politesse, properness, proprieties, propriety, protocol, pudency,
   pudicity, punctilio, quiet good manners, respectability,
   righteousness, rightness, rules of conduct, seemliness, shame,
   social code, social conduct, social convention, social graces,
   social procedures, social usage, solemnity, suitability, urbanity,
   usage