waterline

Short definition from Wiktionary

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.