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,wa'ter•line".
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.