Assassination was a specialised form of murder, in which the victim was a prominent individual — often the leader of a government. A person who carried out an assassination was called an assassin.
The First Doctor told Vicki that it was acceptable to hire an assassin in Roman times, and these assassins were preferably "dumb", i.e. mute, so that they couldn't speak out against their employer. (TV: "All Roads Lead to Rome")
In 64, a centurion hired Ascaris to assassinate the lyre player Maximus Pettulian. After the assassination was carried out, the First Doctor posed as Maximus, making it appear that he had some somehow survived. Believing the Doctor to be Maximus, Ascaris tried to assassinate the Doctor, but was overpowered by the Doctor and Vicki. (TV: The Romans)
In 1400, there was a failed plot to assassinate King Henry IV of England and restore the deposed Richard II to the throne. The conspirators included the author Geoffrey Chaucer and his son Tom. (AUDIO: The Doctor's Tale)
On 28 June 1914, the assassination of Archduke Franz Ferdinand of Austria-Hungary by Gavrilo Princip in Sarajevo, Bosnia triggered a series of events that rapidly escalated into Earth's first global conflict: World War I. Unknown to the general populace, however, the start of the war was influenced by the Nemesis. (PROSE: Human Nature; TV: Silver Nemesis, The Family of Blood)
In 2097, the female Pope visited Mars to anoint the first Martian bishops. This was done to improve relations between Martians and the humans. The Daleks made an assassination attempt. (PROSE: The Gallifrey Chronicles)
Like World War I, World War VI was sparked by an assassination. The assassination of Ingrid Bjarnsdottir, commissioner of the Icelandic Alliance, by the Peking Homunculus began a war between them and the Supreme Alliance of Eastern States led by Magnus Greel in the year 5000. (TV: The Talons of Weng-Chiang; PROSE: Doctor Who and the Talons of Weng-Chiang; AUDIO: The Butcher of Brisbane)
In an alternative timeline, Margaret Thatcher, the Prime Minister of the United Kingdom, was assassinated by her fellow Conservative Party politician Heathcliffe Bower in 1984. However, the proper timeline was restored by the Fifth Doctor. (PROSE: The Assassin's Story)