chief justiciar: an official who acted as regent during the king's absence from England.

In medieval England and Scotland, the Chief Justiciar (latterly known simply as the Justiciar) was a rough equivalent to that of the modern Prime Minister: the Monarch's chief minister. Similar positions existed on the Continent. The term is the English form of the medieval Latin justiciarius or justitiarius ('man of justice', i.e. judge).

formerly a high judicial officer

Same as Justiciary.