What is Capitalis Justiciarius?

Legal Definition
The chief justiciary; the principal minister of state, and guardian of the realm in the king's absence. This office originated under William the Conqueror; but its power was greatly diminished by Magna Charta, and finally distributed among several courts by Edward 1. Spelman ; 3 Bl. Comm. 38
-- Black's Law Dictionary
Legal Definition
The chief justice.
-- Ballentine's Law Dictionary