Treason was a criminal offence of betrayal of one's nation, planet, or other state. The penalty for treason was commonly death. "High Treason" was the most feared form of treason, for it was proven criminal disloyalty to one's government.
Examples of treason among Time Lords were believed rare. For unspecified reasons, Sardon (a member of the High Council with posts in the CIA and the Temporal Scanning Service) had the authority to place his assistant Luco on trial for High Treason after the Second Doctor proved his collaboration with the Players against Gallifrey. It was an unusually severe case as Luco tried multiple times to kill the Doctor and his partner Serena on their assignment for the CIA, invoked forbidden access to the Death Zone, and helped the Players tamper with Time. (PROSE: World Game) The War Chief fled Gallifrey when he was framed for treason by Borusa. (PROSE: Timewyrm: Exodus)
The Doctor seemed to be no stranger to being a high-order criminal. When Lemuel Gulliver said the Master of the Land of Fiction had papers against him for "treason and other capital crimes" The Second Doctor's response was, "Treason again, really?" (TV: The Mind Robber) This bland reaction was echoed in his fourth incarnation, when he was arrested and interrogated by torture in a pre-trial attempt to force a confession to treason for murdering the retiring Lord President. (TV: The Deadly Assassin)
The Fifth Doctor faced treason charges over biodata extraction because it threatened the integrity of the Matrix. The matter was so serious the High Council preferred to act in haste rather than take the time for a full investigation. In this case the charge was weak; the Doctor was sent to be executed in order to spare the lives of billions, but they could not execute him before the proper paperwork was filed. (TV: Arc of Infinity)
The criminal Slitheen family were forced off their home planet, convicted of high treason, as well as fraud and theft, by order of the High Council of Raxas Prime. They were sentenced to death. (TV: The Gift)
During the Human-Hath War on Messaline, General Cobb ordered the Tenth Doctor and Donna Noble imprisoned after the Doctor objected to him using the Source to exterminate the Hath, refusing to "have them spreading treason". (TV: The Doctor's Daughter)
Mavic Chen and his security chief Karlton were found guilty of high treason when a message recorded by Marc Cory revealed that the two men had been conspiring with the Daleks. The sentence was execution, although Chen was already dead when Diksen arrested Karlton. (PROSE: The Mutation of Time)
In 1923, Adolf Hitler was found guilty of treason for leading the Beer Hall Putsch. In 1951 of an alternate timeline, the Seventh Doctor and Ace were arrested on charges of treason against Nazi Germany by Lieutenant Anthony Rupert Hemmings. (PROSE: Timewyrm: Exodus)