As used in this chapter, unless the context requires a different meaning: “Appointing authority” means the General Assembly or the Governor. “Creditable service” means prior service plus membership service, as further defined in and modified by § 51.1-303, for which credit is allowable under this chapter. “Judge” means any justice or judge of a court of record of the Commonwealth, any member of the State Corporation Commission or Virginia Workers’ Compensation Commission, any judge of a district court of the Commonwealth other than a substitute judge of such district court, and any executive secretary of the Supreme Court assuming such position between December 1, 1975, and January 31, 1976. “Normal retirement date” means a member’s sixty-fifth birthday. “Previous systems” means the systems established under the provisions of Chapters 2 (§ 51-3 et seq.) and 2.2 (§ 51-29.8 et seq.) of Title 51, and, in the case of judges of regional juvenile and domestic relations courts, the Virginia Retirement System. “Retirement system” means the Judicial Retirement System. “Service” means service as a judge. “Social security disability benefit” means, with respect to any member, the social security disability benefits to which the member is entitled pursuant to the provisions of the federal Social Security Act as in effect at his date of retirement.