What is Adelantado?

Legal Definition
Adelantado (Spanish pronunciation[aðelanˈtaðo]) was a title held by Spanish nobles in service of their respective kings during the Middle Ages. It was later used as a military title held by some Spanish conquistadores of the 15th, 16th and 17th centuries.

Adelantados were granted directly by the Monarch the right to become governors and justices of a specific region, which they were charged with conquering, in exchange for funding and organizing the initial explorations, settlements and pacification of the target area on behalf of the Crown of Castile. These areas were usually outside the jurisdiction of an existing audiencia or viceroy, and adelantados were authorized to communicate directly with the Council of the Indies.
-- Wikipedia
Legal Definition
In Spanish law. A governor of a province; a president or president judge; a judge having jurisdiction over a kingdom or over certain provinces only. So called from having authority over the judges of those places. Las Partidas, pt. 3, tit 4, 1. 1.
-- Black's Law Dictionary