Definition of "waterline"

(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.  examples

(aviation) A horizontal line indicating the shape of an airfoil.  examples

A line showing where the water has been, usually a line separating dry land and wet areas; a watermark or tidemark.  examples

(cosmetics) The inner rim of the eyelid, just behind the lash line; primarily used in reference to the application of eyeliner.  examples