ambassador:
(plural: ‘ambassadors‘; female: (old usage) ‘ambassadress’; for the right pronunciation of this word, please click here)

His Excellency Mohammed Adam Ismael, the Sudanese ambassador to Yemen
a person, a minister or government official of a high rank, representing his country in another country or an international organisation to maintain good relations with that country and also to look after his/her country’s citizen there and to see that trade agreements are honoured;
an important person who represents an international charitable organisation, a good cause, etc. (without any political or government involvement);
(usually called ‘brand ambassador’) an important person who represents a business firm or company and acts as a model for publicity purpose,

the most popular brand ambassador in India
An ambassador’s main role in a foreign country is to show his/her country there in a very positive light.
There are different types of ambassadors: ‘an ambassador of goodwill’ is the one who represents some organisation or cause and works for it without pay; ‘an ambassador of literacy’ is the one who is well-educated and tries to educated others the importance of literacy’; etc.
According to the popular magazines in India, Priyanka Chopra, former Miss Word and Indian film actress, is the most popular ‘brand ambassador’ representing different large business firms.
Note 2: Though there is this term ‘ambassadress’ for a lady ambassador, it has become ‘old’ and in the present usage, only the word ‘ambassador’ is used for both male and female diplomats.
