California Constitution § 18

Treason against the State consists only in levying war against it, adhering to its enemies, or giving them aid and comfort. A person may not be convicted of treason except on the evidence of two witnesses to the same overt act or by confession in open court.

(Sec. 18 added Nov. 5, 1974, by Prop. 7. Res.Ch. 90, 1974.)