A propeller used to provide a transverse thrust to the bow of a ship and to assist movement in confined spaces.

a small propeller situated in an athwartship tunnel in the underwater hull of a ship; usually located near the bow, the side thrust created by the running propeller improves manoeuvrability, especially during berthing

A bow thruster, also known as a tunnel thruster, is a thruster built into the bow of a ship to enhance its maneuverability. Usually, a bow thruster is just a tunnel below the waterline through the bow. An impeller in the tunnel can create thrust to both ways which makes the ship turn.