A helper; an assistant.

A regimental staff officer, who assists the colonel, or commanding officer of a garrison or regiment, in the details of regimental and garrison duty.

An Officer, normally of the rank of captain, who acts as the commanding officers 'right-hand man' in peace and war, with special responsibility for personnel and discipline.

an Army officer who does administrative work and is responsible for personnel, rules and punishment.

an officer who acts as military assistant to a more senior officer

The senior staff officer of a regiment, the Adjutant handled all administrative details and paperwork. He assigned the duty of both officers and men, such as for guards and detachments.

An adjutant (from the Latin adiutans, present participle of the verb adiutare, "to help"; the Romans actually used adiutor for the noun) is an officer who assists a more senior officer.