A line corresponding to the surface of the water when the vessel is afloat on an even keel; often painted on the hull of a ship WordNet 3.0 & Open English WordNet

The waterline is the line where the hull of a ship meets the surface of the water. Wikipedia

The line where the hull of a ship meets the surface of the water Wikipedia Disambiguation

Nautical term Wikidata

The waterline of a ship. Wiktionary

Waterline of a ship. Wiktionary (translation)

Any one of several lines marked upon the outside of a vessel, corresponding with the surface of the water when she is afloat on an even keel. The lowest line indicates the vessel's proper submergence when not loaded, and is called the light water line; the highest, called the load water line, indicates her proper submergence when loaded. Wiktionary

A line formed by the surface of the water on the hull of a ship when she is afloat; any of a series of short lines marked on the hull to show where the waterline would be under different loadings. Wiktionary

Line on the hull of a ship. Wiktionary (translation)