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]

WordNet (r) 3.0 (2006):

decorum
    n 1: propriety in manners and conduct [syn: decorum,
         decorousness] [ant: indecorousness, indecorum]

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



Bouvier's Law Dictionary, Revised 6th Ed (1856):

DECORUM. Proper behaviour; good order.
     2. Decorum is requisite in public places, in order to permit all
persons to enjoy their rights; for example, decorum is indispensable in
church, to enable those assembled, to worship. If, therefore, a person were
to disturb the congregation, it would be lawful to put him out. The same
might be done in case of a funeral. 1 Mod. 168; 1 Lev. 196 2 Kebl. 124. But
a request to desist should be first made, unless, indeed," when the
necessity of the case would render such precaution impossible. In using
force to restore order and decorum, care must be taken to use no more than
is necessary; for any excess will render the party using it guilty of an
assault and battery. Vide Battery.