A spy was an individual who worked in espionage. A leader of spies was known as a spymaster.
The Doctor and/or their companions were accused of being spies on many occasions, (AUDIO: The Evil One) usually because of their lack of identification and appearing seemingly impossibly without anyone having record of them. Among those who made the accusations based on these factors included Eldred, (TV: The Time Meddler) Odysseus and Cassandra, (TV: The Myth Makers) General Smythe, Major Barrington, Lieutenant Lucke, (TV: The War Games) Brigade Leader Alastair Lethbridge-Stewart, (TV: Inferno) Ortron, (TV: The Monster of Peladon) Tuar, (TV: Planet of the Spiders) the Marshal of Atrios, (TV: The Armageddon Factor) Daisy K (TV: The Happiness Patrol) and Rutan spy Heinz Bruckner. (COMIC: The Instruments of War)
Maxie Smart, a secret agent, was once pictured by TV 21 wearing spy braces. (PROSE: Time Barrier Busted)
Cyclops worked as a spy for the Greeks during the Trojan War. (TV: The Myth Makers) However, according to another source, the spy was Homer himself. (PROSE: The Myth Makers)
Lieutenant Symonds initially believed that Ian Chesterton was a Russian spy during the First Afghan War before learning he was English. (PROSE: Mire and Clay)
Kalik believed that Vorg was a spy. (TV: Carnival of Monsters)
William Shakespeare worked as a spy for King James I. (PROSE: The Empire of Glass)
When he was brought before Wrack, Turlough accused the Doctor of being a spy. (TV: Enlightenment)
Miles Barrington Smythe was a Russian spy in 1963 London. (TV: The Cambridge Spy)
A spy stole a top secret file from UNIT HQ. (COMIC: Levitation)