A leader or official — military commander, court official, or royal prince. The Messianic title 'Prince of Peace' (Isaiah 9:6) uses this word as sar-shalom.
Authority structures in Israel used sar at every level: commanders of thousands, princes of tribes, officials in the court. The divine Warrior-Prince of Joshua 5 is the pre-incarnate Christ.
Isaiah 9:6's compound title — sar-shalom — makes peace the defining characteristic of Messiah's rule. He is not a war-prince who brings peace after conquest; he IS the Prince of Peace, whose kingdom IS shalom.