In law, treason is the crime that covers some of the more serious acts of disloyalty to one's sovereign or nation. Historically, treason also covered the murder of specific social superiors, such as the murder of a husband by his wife (treason against the king was known as high treason and treason against a lesser superior was petit treason). A person who commits treason is known in law as a traitor.

Oran's Dictionary of the Law (1983) defines treason as: "...1...citizen's actions to help a foreign government overthrow, make war against, or seriously injure the nation." In many nations, it is also often considered treason to attempt or conspire to overthrow the government, even if no foreign country is aided or involved by such an endeavour.

Outside legal spheres, the word "traitor" may also be used to describe a person who betrays (or is accused of betraying) their own political party, nation, family, friends, ethnic group, religion, social class, or other group to which they may belong. Often, such accusations are controversial and disputed, as the person may not identify with the group of which they are a member, or may otherwise disagree with the group leaders making the charge. See, for example, race traitor.

At times, the term "traitor" has been levelled as a political epithet, regardless of any verifiable treasonable action. In a civil war or insurrection, the winners may deem the losers to be traitors. Likewise the term "traitor" is used in heated political discussion typically as a slur against political dissidents, or against officials in power who are perceived as failing to act in the best interest of their constituents. In certain cases, as with the German Dolchstoßlegende, the accusation of treason towards a large group of people can be a unifying political message.

Murder is now generally considered the worst of crimesFact: date=December 2008, but in the past, treason was thought of as worse. In English law high treason was punishable by being hanged, drawn and quartered (men) or burnt at the stake (women), the only crime which attracted those penalties (until the Treason Act 1814). The penalty was used by later monarchs against people who could reasonably be called traitors, although most modern jurists would call it excessive. Many of them would now just be considered dissidents.

In William Shakespeare's play King Lear (circa 1600), when the King learns that his daughter Regan has publicly dishonoured him, he says They could not, would not do 't; 'tis worse than murder: a conventional attitude at that time. In Dante Alighieri's Inferno, the ninth and lowest circle of Hell is reserved for traitors; Judas Iscariot, who betrayed Jesus, suffers the worst torments of all: being constantly gnawed at by one of Lucifer's own three mouths. His treachery is in fact so notorious that his name has long been synonymous with traitor, a fate he shares with Benedict Arnold, Marcus Junius Brutus (who too is depicted in Dante's Inferno, suffering the same fate as Judas along with Cassius Chaerea), and Vidkun Quisling. Indeed, the etymology of the word traitor originates with Judas' handing over of Jesus to the Roman authorities: the word is derived from the Latin traditorem which means "one who delivers."