The code of polite behavior in social or professional settings.

• Unwritten norms guiding conduct within a group or profession.

社交或职业场合中的礼貌行为规范。

• 群体内部的约定俗成的行为准则。

A label indicating a letter is to go by airmail.

• Term used in philately.

贴在信件上表示航空邮寄的标签。

• 邮票学中的专用术语。