an important official who works in a foreign country representing his or her own country there, and who is officially accepted in this position by that country

The UK's ambassador in Moscow has refused to comment. 英国驻莫斯科大使已拒绝发表评论。

She's a former ambassador to the United States. 她是前驻美国大使。

Late last night, the French ambassador was summoned to the Foreign Office to discuss the crisis. 昨晚深夜法国大使被外交部召见，商讨这起危机事宜。

At that time, Sir Jeremy Greenstock was the British UN ambassador (= represented the United Kingdom at the United Nations).