noun

• (uncountable) Appropriate social behavior.
  "Everyone was assembled in the dining-room. Under the circumstances, we were naturally not a cheerful party. The reaction after a shock is always trying, and I think we were all suffering from it. Decorum and good breeding naturally enjoined that our demeanour should be much as usual, yet I could not help wondering if this self-control were really a matter of great difficulty. There were no red eyes, no signs of secretly indulged grief."
• (countable) A convention of social behavior.
  "In the architecture and embellishments of the chamber, the evident design was to dazzle and astound. Little attention had been paid to the decora of what is technically called “keeping,” or to the proprieties of nationality. The eye wandered from object to object, and rested upon none; neither the “Grotesques” of the Greek painters, nor the sculptures of the best Italian days, nor the huge carvings of untutored Egypt."