Emily Dickinson Lexicon

DE-CO'RUM, n. [L. from deceo, to become. See Decency.]

1. Propriety of speech or behavior; suitableness of speech and behavior, to one's own character, and to the characters present, or to the place and occasion; seemliness; decency; opposed to rudeness, licentiousness, or levity. To speak and behave with decorum is essential to good breeding.
2. In architecture, the suitableness of a building, and of its parts and ornaments, to its place and uses.