An Ambassador is an official government representative who is tasked to represent his government in another country in diplomatic issues. Usually they are based in the capitol of the country they are assigned to.
In 1967 Morris Monroe was the official Ambassador for the United States to the country of West Germany. UNITY discovered that a terrorist organization called H.A.R.M. had targeted Ambassador Monroe for Assassination and sent Bruno Lawrie and Cate Archer to protect him. Despite their best efforts, Dmitrij Volkov was able to kill Ambassador Monroe on his way to the airport in Marrakech, Morocco.
When The Director attempted to blackmail the Soviet Union over the planned invasion of Khios in 1968, the Soviets responded that they would be contacting their Ambassador in Washington D.C. The Director doubted that the Americans would believe the Soviets.
Ambassador on Wikipedia