an official given the authority and responsibility to act as a deputy/substitute for a superior official; a commissioned officer in a force such as the army, navy, or fire/police department, whose rank varies based on the force;
e.g. He asked his top lieutenant to look into the matter because he was too busy to do it himself.
(noun)
an aid/assistant or agent;
e.g. Harvey's best lieutenant was working on the legal deal.