Here's Wikipedia on Treason:
'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.'
Where we are today:
trump and the GOP have implemented several United States policies and practices which harm the National Security of the United States and also have a direct and positive impact on Russia's interests.
These actions are listed in the video clip from Rachel Maddow: http://www.msnbc.com/rachel-maddow/watch/what-would-russia-want-from-a-pliant-us-leader-1003069507592