noun

• The forms required by a good upbringing, or prescribed by authority, to be observed in social or official life; observance of the proprieties of rank and occasion; conventional decorum; ceremonial code of polite society.
• The customary behavior of members of a profession, business, law, or sports team towards each other.
• July 15 2012, Richard Williams, Tour de France 2012: Carpet tacks cannot force Bradley Wiggins off track, Guardian Unlimited:
• A label used to indicate that a letter is to be sent by airmail.