The Geoid is a surface of equal gravitational potential as
would, for example, be characterised by the interface between to inviscid fluids
(ie. the mean elevation of sea surface).
The Geoid is measured as the anomaly from a hypothetical
question : is there any correlation between the surface plates
and the long-wavelneth geoid? (hint compare the hypsography of the topography
with that of the geoid).