chief

the head of a police department

the head of a fire department

the leader of a tribe

the leader of a village

the head of a division of an organization

the head of a branch of an organization

a fixed title for the head of the police

the most senior officer in an army, navy, or air force

to start being the chief

to officially choose someone to be the chief

to name someone as the new chief

the person with the most important position in a company or organization

the most important reason

the main reason for something happening

the main competitor or opponent

the main person or group one is fighting against