legatus

Legatus, legati, m., second declension, and originally a perfect participle: the man who has been deputed. Two careers live in the one noun. In civil life he is an envoy carrying somebody else's authority, sent by a senate, a city or a king. In the army he is the commander's deputy, a senator delegated to a legion, which is why Caesar's narrative is full of them.

Medieval Latin keeps the civil sense and raises the stakes: a papal legatus a latere is a cardinal sent out with the pope's own powers rather than his own. Do not confuse him with legatum, the neuter noun, which is a legacy in a will - the same verb legare, to depute or bequeath, going in a different direction. English legate, legation and delegate all follow.