There are several definitions for "ambassador". The position may be an official one or an informal one.
am·bas·sa·dor (ăm-băs'ə-dər, -dôr')
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.
A diplomatic official heading his or her country's permanent mission to certain international organizations, such as the United Nations.
An authorized messenger or representative.
An unofficial representative: ambassadors of goodwill.