An ambassador was a diplomatic representative commissioned by a faction or species.
During World War I, Bertie Stopford was the United Kingdom ambassador to the Russian Empire. However, he was forced to return to the United Kingdom in the Russian Revolution of 1917. (PROSE: The Wages of Sin)
Erato was sent as an ambassador to negotiate a trade agreement with Chloris. His own planet, Tythonus, was in need of chlorophyll, which the jungle planet Chloris had in abundance. Chloris had a deficiency of metal, which Tythonus had in abundance. However, Lady Adrasta had a monopoly on metal and, in order to retain this, imprisoned the ambassador. He was freed by the Fourth Doctor, Adrasta was overthrown, and an agreement in line with original intention was reached. (TV: The Creature from the Pit)
Rahnius was an ambassador for the Galactic Alliance. He was killed by a member of the Slitheen family, allowing the murderer to adopt his reputable identity. (TV: From Raxacoricofallapatorius With Love)
In order to make their demands for ten percent of Earth's children, the 456 chose to send an ambassador to conduct relations with the humans. Disregarding the entitlement of ambassadors to safe conduct, Torchwood killed the representative with a sound wave. (TV: Children of Earth: Day Three)
Ambassadors from the City State of Binding Light were present on Platform One to witness the destruction of Earth. Following manipulation of the platform's protective field, one of the ambassadors was killed. (TV: The End of the World)