What is Treason?

Legal Definition
In law, treason is the crime that covers some of the more extreme acts against one's nation or sovereign. Historically, 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 against the king was known as high treason and treason against a lesser superior was petty treason. A person who commits treason is known in law as a traitor. Oran's Dictionary of the Law (1983) defines treason as "...[a]...citizen's actions to help a foreign government overthrow, make war against, or seriously injure the [parent nation]." In many nations, it is also often considered treason to attempt or conspire to overthrow the government, even if no foreign country is aiding or involved by such an endeavor.

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 German Dolchstoßlegende, the accusation of treason towards a large group of people can be a unifying political message. Treason is considered to be different and on many occasions a separate charge from 'Treasonable Felony' in many parts of the world.
-- Wikipedia
Legal Definition
The offence of betraying one’s own country by attempting to overthrow the government through waging war against the state or materially aiding its enemies. Also termed high treason; alta proditio.

According to the United States Constitution, Article III, § 3, “Treason against the United States, shall consist only in levying war against them, or in adhering to their Enemies, giving them Aid and Comfort. No Person shall be convicted of Treason unless on the Testimony of two Witnesses to the same overt Act, or on Confession in open Court.”

Under federal statute, a person guilty of treason against the United States “shall suffer death, or shall be imprisoned not less than five years and fined under this title but not less than $10,000; and shall be incapable of holding any office under the United States.” 18 U.S.C. § 2381.
Legal Definition
Crim. law. This word imports a betraying, treachery, or breach of allegiance. 4 Bl. Com. 75.

2. The constitution of the United States, art. 3, s. 3, defines treason against the United States to consist only in levying war (q. v.) against them, or in adhering to their enemies, giving them aid or comfort. This offence is punished with death. Act of April 30th, 1790, 1 Story's Laws U. S. 83. By the same article of the constitution, no person shall be convicted of treason, unless on the testimony of two witnesses to the same overt act, or on confession in open court. Vide, generally, 3 Story on the Const. ch. 39, p. 667; Serg. on the Const. ch. 30; United States v. Fries, Pamph.; 1 Tucker's Blackst. Comm. Appen. 275, 276; 3 Wils. Law Lect. 96 to 99; Foster, Disc. I; Burr's Trial; 4 Cranch, R. 126, 469 to 508; 2 Dall. R. 246; 355; 1 Dall. Rep. 35; 3 Wash. C. C. Rep. 234; 1 John. Rep. 553 11 Johns. R. 549; Com. Dig. Justices, K; 1 East, P. C. 37 to 158; 2 Chit. Crim. Law, 60 to 102; Arch. Cr. Pl. 378 to 387.
-- Bouviers Law Dictionary
Legal Definition
The offence of attempting to overthrow the government of the state to which the offender owes allegiance; or of betraying the state into the hands of a foreign power. Webster. In England, treason is an offence particularly directed against the person of the sovereign, and consists

(1) in compassing or imagining the death of the king or queen, or their eldest sen and heir;

(2) in violating the king's companion, or the king's eldest daughter unmarried, or the wife of the king's eldest son and heir;

(3) in levying war against the king in his realm;

(4) in adhering to the king's enemies in his realm, giving to them aid and comfort in the realm or elsewhere, and

(5) slaying the chancellor, treasurer, or the king's justices of the one bench or the other, justices in eyre, or justices of assize, and all other justices assigned to hear and determine, being in their places doing their offices. 4 Steph. Comm. 185-193; 4 Bl. Comm. 76-84. "Treason against the United States shall consist only in levying war against them, or in adhering to their enemies, giving them aid and comfort." U. S. Co.nst. art. 3, § 3, cl. 1. See Young v. U. S., 97 U. S. 62, 24 In Ed. 992; U. S. v. Bollman, 1 Cranch, C. C. 373, Fed. Cas. No. 14,622; U. Sv. Greathouse, 4 Sawy. 457, 2 Abb. U. S. 364, Fed. Cas. No. 15,254; U. S. v. Hanway, 2 Wall. Jr. 139, Fed. Cas. No. 15,299; U. S. v. Hoxie, 1 Paine, 265, Fed. Cas. No. 15,407; U. S. v. Pryor, 3 Wash. C. C. 234, Fed. Cas. No. 16,096.
See also
-- Black's Law Dictionary
Legal Definition
A breach of allegiance by one who owes allegiance, perpetual or temporary. See 5 Wheat. (U. S.) 76, 5 L. Ed. 37. Under the United States constitution, levying war against the United States, or adhering to their enemies, giving them aid and comfort. See 4 Sawy. 457, 2 Abb. (U. S.) 364, 26 Fed. Cas. (U. S.) 18. See, also, Petit treason.
-- Ballentine's Law Dictionary