1. Naut. the part of the outside of a ship's hull that is just at the water level.
2. Naval Archit. 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. Cf. load line, Plimsoll line .
3. the line in which water at its surface borders upon a floating body.
4. See water level (def. 2).
5. 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.
6. a pipe, hose, tube, or other line for conveying water.
Also, waterline .
[ 1615-25 ]