ambassador

1. A diplomatic official of the highest rank appointed and accredited as representative in residence by one government or sovereign to another, usually for a specific length of time.
2. A diplomatic official heading his or her country's permanent mission to certain international organizations, such as the United Nations.
3. An authorized messenger or representative.
4. An unofficial representative: ambassadors of goodwill.

[Middle English ambassadour, from Old French ambassadeur, from Medieval Latin ambactia, mission, from Latin ambactus, servant, ultimately of Celtic origin.]