What is Treason-felony?

Legal Definition
under the English statute 11 & 12 Viet. c. 12, passed in 1848, is the offence of compassing, devising, etc., to depose her majesty from the crown; or to levy war in order to intimidate either house of parliament, etc., or to stir up foreigners by any printing or writing to invade the kingdom. This offence is punishable with penal servitude for life, or for any term not less than five years, etc., under statutes 11 & 12 Vict. c. 12, § 3; 20 & 21 Vict. c. 3, § 2; 27 & 28 Vict. c. 47, § 2. By the statute first above mentioned, the government is enabled to treat as felony many offenses which must formerly have been treated as high treason. Mozley & Whitley.
-- Black's Law Dictionary