Distance from the center of the Earth to the sea surface
The eustatic sea level (from Greek εὖ "eû", "good" and στάσις "stásis", "standing") is the distance from the center of the Earth to the sea surface. An increase of the eustatic sea level can be generated by decreasing glaciation, increasing spreading rates of the mid-ocean ridges or increasing the number of mid-oceanic ridges. Conversely, increasing glaciation, decreasing spreading rates or fewer mid-ocean ridges can lead to a fall in the eustatic sea level.
Changes in the eustatic sea level lead to changes in accommodation and therefore affect the deposition of sediments in marine environments.
Overview.
The eustatic (global) sea level is the volume of Earth's oceans. This is not a physical level but instead represents the sea level if all of the water in the oceans were contained in a single basin. Eustatic sea level is not relative to local surfaces, because relative sea level depends on many factors – including tectonics, continental rise and subsidence. Eustatic sea level follows the "bathtub approach" which describes the ocean as a single bathtub. One can add or remove water and Earth's oceans will gain or lose water globally. Differences in eustatic sea level over time stem from three main factors: 
References.
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