A thief was an individual who took property which did not belong to them.
The First Doctor, Barbara, Ian, and Susan met Robin Hood, Little John, and Will Scarlet. The three were members of a group of outlaws who stole from both the rich and poor as a source of income. (PROSE: The Thief of Sherwood)
While in Napoleonic Russia, the First Doctor, Steven, and Dodo encountered a shapeshifter they called the Shape Thief. The Shape Thief changed into the Doctor and tried to steal the TARDIS. (AUDIO: Mother Russia)
The Third Doctor incorporated an anti-thief device into Bessie. The Doctor used the device at least once to aid in the recovery of the Recovery 7 landing capsule which had been stolen by two thieves. (TV: The Ambassadors of Death)
Liz Shaw, when working at the University of Rome on a bust of Nero with a man named Jonas Smythe (the Fifth Doctor), encountered a thief who tried to steal the bust. (PROSE: Flashpoint)
When he tried to visit his friends Lord and Lady Tubbs, the Tenth Doctor discovered a thief had tried to steal an ancient horologe, dropped it, and transported those in the immediate vicinity into a different kind of time. (COMIC: 13 O'Clock)
River Song once claimed that "An archaeologist is just a thief with patience." (TV: The Husbands of River Song)