The oblate spheroid, or oblate ellipsoid, is an ellipsoid of revolution obtained by rotating an ellipse about its shorter axis. It is the regular geometric shape that most nearly approximates the shape of the Earth.
The earth is flattened at the poles and bulged at the center. Geodesy represents the shape of the earth with an oblate spheroid.
Even though our planet is a sphere, it is not a perfect sphere. Because of the force caused when Earth rotates, the North and South Poles are slightly flat. Earth's rotation, wobbly motion and other forces are making the planet change shape very slowly, but it is still round.
First and foremost, the Earth itself isn't a sphere. It is closer to an ellipsoid, being flattened at the poles by the centrifugal force of its own rotation. But the planet isn't an ellipsoid either, because of topography. Mountains and valleys are asymmetrical distributions of mass.
A planet is round because of gravity. A planet's gravity pulls equally from all sides. Gravity pulls from the center to the edges like the spokes of a bicycle wheel. This makes the overall shape of a planet a sphere, which is a three-dimensional circle.
Our planet's shape, caused by the flattening at the poles, is called an oblate spheroid. Those numbers make Earth just slightly bigger than Venus, whose equatorial radius is about 3,761 miles (6,052 km).
The Earth isn't a perfect sphere, however. It is an oblate spheroid, meaning it stretches out a little around the Equator in a form called an equatorial bulge. The Equator is around 40,075 kilometers (24,901 miles) in circumference.