The shape that Earth's surface would form if it were only affected by gravity and ocean water, ignoring mountains and other irregular features.
From geo- (Earth) + -oid (resembling or having the form of), derived from Greek eidos (form). Scientists coined this term in the 19th century to describe an idealized Earth shape for measurements.
The geoid is mind-bending—Earth's true gravitational surface doesn't match any simple sphere or ellipsoid because dense rock regions pull stronger and create bumps in the gravitational field, which is why GPS satellites have to account for these invisible 'mountains' of gravity that our eyes can't see.
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