A medieval-style helmet that completely encloses the head. It offers absolute protection at the expense of being cumbersome and limiting vision greatly. This is the classic knightly armor and is generally build around the entire suit of armor. This is separate from a helmet with a built-in face mask since that is the addition of a component to the helmet while the helm is one-piece.
For depictions of the steering mechanism of a ship, use the ship's_wheel tag.