Geoid
The hypothetical shape of Earth's surface defined by an equipotential surface of Earth's gravity field matching mean ocean sea level in the absence of tides and winds.
Governing formula
Equipotential Surface V_gravity = Constant
SI unit
Meters (Geoid Undulation Height)
In depth
Because Earth's internal mass distribution is irregular and non-uniform, the geoid is an irregular, lumpy shape rather than a perfect ellipsoid. Ocean water flows along the geoid equipotential, making the geoid the true zero reference for elevation.
Examples in the real world
Satellite altimetry mapping geoid undulations (+80 m to -100 m relative to reference ellipsoid) to identify underwater gravity anomalies.