etiquette

Short definition from Wiktionary

Noun

1. 1
   The manners or decent behaviour to be observed in social or professional life; conventional decorum; the ceremonial code of polite society.
2. 2
   The customary behavior of members of a profession, business, law, or sports team towards each other.
3. 3
   A label used to indicate that a letter is to be sent by airmail.