Plural form; irregular rounded shapes or bodies resembling the Earth, or mathematical models of Earth's actual gravitational shape (distinct from a sphere).
From Greek 'geo-' (earth) and '-oid' (resembling). Popularized in geodesy and geophysics in the 20th century as scientists mapped gravitational variations.
The 'geoid' is Earth's true shape when you account for gravity—it's not a sphere or even a perfect ellipsoid, but an irregular bumpy surface that follows gravitational pull. It's what a perfectly still ocean would become if there were no land.
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