Wikipedia
The Senators of the College of Justice are judges of the College of Justice, a set of legal institutions involved in the administration of justice in Scotland. There are three types of Senator: Lords of Session (judges of the Court of Session); Lords Commissioner of Justiciary (judges of the High Court of Justiciary); and the Chairman of the Scottish Land Court. Whilst the High Court and Court of Session historically maintained separate judiciary, these are now one and the same, and the term, Senator, is almost exclusively used in referring to the judges of these courts.
Senators of the College use the title Lord or Lady along with a surname or a territorial name. Note, however, that some Senators have a peerage title, which would be used instead of the senatorial title. All Senators of the College have the honorific, The Honourable, before their titles, while those who are also Privy Counsellors or peers have the honorific, The Right Honourable. Senators are made Privy Counsellors upon promotion to the Inner House.
Under section 11 of the Treason Act 1708, it is treason to kill a Senator of the College of Justice "sitting in Judgment in the Exercise of their Office within Scotland".