Treason is defined in Article III, Section 3 of the U.S. Constitution.

Treason against the United States shall consist only in levying war against them, or in adhering to their enemies, giving them aid and comfort.

Levying war: Engaging in armed conflict against the United States.

Adhering to enemies: Providing support or assistance to enemies of the United States.

The definition also stipulates that a conviction of treason requires the testimony of two witnesses to the same explicit act or a confession in open court.