What is Decorum?

Legal Definition
Decorum (from the Latin: "right, proper") was a principle of classical rhetoric, poetry and theatrical theory that was about the fitness or otherwise of a style to a theatrical subject. The concept of decorum is also applied to prescribed limits of appropriate social behavior within set situations.
-- Wikipedia
Legal Definition
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.
-- Bouviers Law Dictionary