waterline

Etymology

water + line

noun

1. (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.
2. (aviation) A horizontal line indicating the shape of an airfoil.
3. A line showing where the water has been, usually a line separating dry land and wet areas; a watermark or tidemark.
4. (cosmetics) The inner rim of the eyelid, just behind the lash line; primarily used in reference to the application of eyeliner.