waterline

noun

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