waterline

Waterline: the line on a ship's hull which is level with the surface of the water on which it floats. The location of this line on the hull will change depending on the ship's load. As it takes on more cargo, the waterline will rise on the hull as it is pushed further down, displacing more water. The waterline has implications for the placement of insulation, structural bracing, and (in the case of warships) various types of armor by marine architects.

Waterline: Also used in reference to aircraft. Used as a common reference to specify the vertical location of objects installed in an aircraft. It is often abbreviated in technical drawings as WL.

Example: If the floor of this aircraft is located at WL 208.60, and this cabinet is 30" tall, then the top of the cabinet is at WL 238.60.