Treason is the crime of attacking a state authority to which one owes allegiance. This typically includes acts such as participating in a war against one's native country, attempting to overthrow its government, spying on its military, its diplomats, or its secret services for a hostile and foreign power, or attempting to kill its head of state. A person who commits treason is known in law as a traitor.

Historically, in common law countries, treason also covered the murder of specific social superiors, such as the murder of a husband by his wife or that of a master by his servant. Treason (for example disloyalty) against one's monarch was known as high treason and treason against a lesser superior was petty treason. As jurisdictions around the world abolished petty treason, "treason" came to refer to what was historically known as high treason.

At times, the term traitor has been used 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 Dolchstoßlegende (Stab-in-the-back myth), the accusation of treason towards a large group of people can be a unifying political message.

United States.

The offense of treason exists at both federal and state levels. The federal crime is defined in the Constitution as either levying war against the United States or adhering to its enemies, and carries a sentence of death or imprisonment and fine.

In the 1790s, opposition political parties were new and not fully accepted. Government leaders often considered their opponents to be traitors. Historian Ron Chernow reports that Secretary of the Treasury Alexander Hamilton and President George Washington "regarded much of the criticism fired at their administration as disloyal, even treasonous, in nature." When the undeclared Quasi-War broke out with France in 1797–98, "Hamilton increasingly mistook dissent for treason and engaged in hyperbole." Furthermore, the Jeffersonian opposition party behaved the same way. After 1801, with a peaceful transition in the political party in power, the rhetoric of "treason" against political opponents diminished.