A Sag

1. The state of sinking or bending; a droop. https://en.wikipedia.org/wiki/Lens_sag
2. The difference in elevation of a wire, cable, chain or rope suspended between two consecutive points.
3. The difference in height or depth between the vertex and the rim of a curved surface, specifically used for optical elements such as a mirror or lens. Sag (geology) - Wikipedia Sag pond - Wikipedia