- the line to which the surface of the water comes on the side of a ship or boat
- any of several horizontal lines marked on the side of a ship to indicate the level to which it sinks when fully or partly loaded or when not loaded, and to aid in achieving a proper trim
- a pipe, tube, etc. for conveying water
- watermark (sense )
- (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.
- (aviation) A horizontal line indicating the shape of an airfoil.
- A line showing where the water has been, usually a line separating dry land and wet areas; a watermark or tidemark.
- (cosmetics) the inner rim of the eyelid, just behind the lash line; primarily used in reference to the application of eyeliner.