From BiblePortal Wikipedia

Webster's Dictionary [1]

(1): ( n.) An official assistant given to a general or to the governor of a province.

(2): ( n.) An ecclesiastic representing the pope and invested with the authority of the Holy See.

(3): ( n.) An ambassador or envoy.

(4): ( n.) Under the emperors, a governor sent to a province.

Charles Buck Theological Dictionary [2]

A cardinal, or bishop whom the pope sends as his ambassador to sovereign princes.

The Nuttall Encyclopedia [3]

The title of the Pope's representative or ambassador; in medieval times this office was attached to certain bishoprics, and the bishops were styled legati nati; besides these there were legati a latere , generally cardinals, and legati missi , or nuncios specially appointed; legates used to claim full papal jurisdiction within their provinces, which caused many disputes; now they are ambassadors for spiritual purposes at Roman Catholic Courts—Vienna, Münich, Madrid, Lisbon, and Paris—and do not interfere with the authority of the bishops.