Position of of sea surface relative to a fixed datum near the sea floor which takes into account two components: eustasy and vertical movement of the sea floor (tectonism and/or sediment compaction) (Posamentier, and Allen 1999).

References
Posamentier, H.W., Allen, G.P., 1999. Siliciclastic sequence stratigraphy: concepts and applications. SEPM Concepts in Sedimentology and Paleontology no. 7, 210 p
Posamentier, H.W., Jervey, M.T., Vail, P.R., 1988, Eustatic controls on clastic deposition. I. Conceptual framework. In: Wilgus, C.K., Hastings, B.S., Kendall, C.G.St.C., Posamentier, H.W., Ross, C.A., Van Wagoner, J.C. (Eds.), Sea Level Changes––An Integrated Approach, vol. 42. SEPM Special Publication, pp. 110– 124.