What is High Treason?

Legal Definition
Treason is criminal disloyalty. Historically, in common law countries, high treason is treason against the state. It was differentiated from petty treason (or petit treason), which was treason against a lesser lawful superior (such as a servant killing his master). Petty treason was restricted to cases of homicide in 1351, and came to be considered a more serious degree of murder.

As common law jurisdictions around the world abolished petty treason, the concept of high treason gradually faded, and today use of the word "treason" generally refers to what was historically known as high treason. In Canadian law, however, there are still two separate offences of treason and high treason, but both of these, in fact, fall in the historical category of high treason. In Canada, the main difference in law between treason and high treason depends on whether the nation is at war. In nations without a common law legal system, the distinction between high and petty treason did not exist.

High treason is criminal disloyalty to one's government. 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 are perhaps the best known examples of high treason. High treason requires that the alleged traitor have obligations of loyalty in the state he or she betrayed, but this will usually be satisfied by being present in the state at the time of the offence, or being a citizen of the state if abroad. Foreign spies, assassins, and saboteurs, though not suffering the dishonor associated with conviction for high treason, may still be tried and punished judicially for acts of espionage, assassination, or sabotage, though in contemporary times, foreign spies are usually repatriated in exchange for spies of the mentioned nation held by another nation. High treason is considered a very serious – often the most serious possible – crime, by the civil authorities. A conviction, by a Canadian court, for high treason, results in a mandatory life sentence (albeit with the possibility of parole after 25 years).

Until the 19th century, counterfeiting coins was high treason in the United Kingdom as it was considered to be an act of debasing the currency of The Crown.
-- Wikipedia
Legal Definition
English law. Treason against the king, in contradistinction with petit treason, which is the treason of a servant towards his master; a wife towards her husband; a secular or religious man against his prelate. See Petit treason; Treason.
-- Bouviers Law Dictionary
Legal Definition
In English law. Treason against the king or sovereign, as distinguished from petit or petty treason, which might formerly be committed against a subject. 4 Bl. Comm. 74, 75; 4 Steph. Comm. 183, 184, note.
-- Black's Law Dictionary
Legal Definition
Treason against the king or the government.
-- Ballentine's Law Dictionary