What is the meaning of Waterline?

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

A horizontal line indicating the shape of an airfoil.

A line showing where the water has been.

The line formed by where the water level of a body of water currently is: where the water's surface currently meets the bank; it may move higher or lower depending on the weather, tide, or season.

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

Source: wiktionary.org