- Nautical. the part of the outside of a ship’s hull that is just at the water level.
- Naval Architecture. any of a series of lines on the hull plans of a vessel representing the level to which the vessel is immersed or the bottom of the keel.Compare load line, Plimsoll line.
- the line in which water at its surface borders upon a floating body.
- water level(def 2).
- Also called watermark. a line indicating the former level or passage of water: A water line all around the cellar served as a reminder of the flood.
- a pipe, hose, tube, or other line for conveying water.