Noun
(nautical) 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.
(aviation) A horizontal line indicating the shape of an airfoil.
A line showing where the water has been, usually a line separating dry land and wet areas; a watermark or tidemark.
(cosmetics) The inner rim of the eyelid, just behind the lash line; primarily used in reference to the application of eyeliner.
Source: en.wiktionary.orgAbove the waterline, the hull gradually narrows to compensate for the weight of the guns and to make boarding more difficult. Source: Internet
A heavily rockered boat curves more, shortening its effective waterline. Source: Internet
For horizontal attacks, the pilot was to "aim at the middle of the vessel, slightly higher than the waterline" or to "aim at the entrance to the aircraft hangar, or the bottom of the stack" if the former was too difficult. Source: Internet
As the model made the turn, the breeze in the upper works forced it to heel more than at calm, forcing the main deck gun ports below the waterline and foundering the model within a few seconds. Source: Internet
Also the wing on wing configuration cannot be heeled over to reduce waterline whereas the spinnaker configuration can be. Source: Internet
A major problem of the reconstruction was that the ship's increased draft meant that their waterline armor belt was almost completely submerged with any significant load. Source: Internet