What is Regius Professor?

Legal Definition
A Regius professor is a university professor with royal patronage or appointment. They are a unique feature of academia in the British Isles. The first Regius professorship was in the field of medicine, and founded by the Scottish King James IV at Aberdeen University in 1497. Regius chairs have since been instituted in various universities, in disciplines judged to be fundamental and for which there is a continuing and significant need. Each was established by a British monarch, and following proper advertisement and interview through the offices of the university and the national government, the current monarch still appoints the professor (except for those at the University of Dublin in Ireland, which left the United Kingdom in 1922). This royal imprimatur, and the relative rarity of these professorships, means a Regius chair is prestigious and highly sought-after.

Regius professors are traditionally addressed as "Regius" and not "Professor". Glasgow University currently has the highest number of extant Regius chairs, at thirteen.
-- Wikipedia
Legal Definition
A royal professor or reader of lectures founded in the English universities by the king. Henry VIII. founded in cach of the universities five professorships, viz., of divinity, Greek, Hebrew, law, and physic. Cowell.
-- Black's Law Dictionary