Eustasy refers to global changes in sea level caused by variations in the volume of water in the oceans, typically due to the melting or formation of ice sheets and glaciers. Eustatic changes are crucial in understanding past climate change, sedimentary sequences, and the formation of coastal and marine landforms.
Source Miller, K. G., Fairbanks, R. G., & Mountain, G. S. (1987). “Tertiary Oxygen Isotope Synthesis, Sea Level History, and Continental Margin Erosion.” Paleoceanography, 2(1), 1-19.