Noun
geoid (plural geoids)
(geography, geodesy) The shape, extending through landmasses (continents, etc.), that the surface of the oceans of the Earth would take under the influence of the Earth's gravity and rotation alone, disregarding other factors such as winds and tides; that is, a surface of constant gravitational potential at zero elevation.
Also, the true vertical at a point at a specific time is influenced by tidal forces, which the theoretical geoid averages out. Source: Internet
In general the true vertical at a point on the surface does not exactly coincide with either the normal to the reference ellipsoid or the normal to the geoid. Source: Internet
In effect it defined the geoid (mean sea level) in terms of a particular level of gravitational time dilation relative to a notional observer located at infinitely high altitude. Source: Internet
Points on the real surface of the earth are usually above or below this idealized geoid surface and here the true vertical can vary slightly. Source: Internet
Starting from Julian Date 2443144.5 (1 January 1977 00:00:00), corrections were applied to the output of all participating clocks, so that TAI would correspond to proper time at mean sea level (the geoid). Source: Internet
The comparison of the observer's clock against TT depends on the observer's altitude: they will match on the geoid, and clocks at higher altitude tick slightly faster. Source: Internet