1. A figure resembling the shape of the Earth: The students in Mrs. Tree's class made a geoid, a perfect representation of the globe.
2. A slightly flattened sphere which is in the shape of the Earth: A geoid is used in calculating the precise measurements of points on the Earth's surface.
3. The theoretical surface that a planet-wide ocean would take if there were no tides or currents: A geoid, or a hypothetical surface of the Earth, would exist if a cross section were taken at sea level, or the figure of the Earth considered as a sea-level surface extended continuously over the entire Earth's surface.