More generally: any officer in command of a body of approximately one hundred soldiers; the leader of a unit or body of militia. Compare "centurion". Now rare (historical in later use).
Specifically. With reference to the Channel Islands. Originally: an officer having charge of a local militia (now historical). In later use in Jersey: a senior member of the organization responsible for law enforcement on the island; (now only) a senior member of the Jersey Honorary Police force, subordinate to the Connétable of the parish.