Ellipticity, in the context of planets, refers to the measure of how much a planet deviates from a perfect sphere in shape. It quantifies the degree of elongation or flattening of a planet's shape due to its rotation and internal forces.

Planets are not perfect spheres due to various factors such as their rotation and internal structure. The gravitational forces and the rotation speed of a planet cause it to bulge at the equator and flatten at the poles. The ellipticity of a planet is a measure of this deviation from a spherical shape.

Ellipticity is typically represented by a dimensionless quantity called the flattening factor or the eccentricity. It is calculated as the difference between the equatorial radius and the polar radius, divided by the equatorial radius. The result indicates the relative amount of flattening or elongation of the planet.

The ellipticity of a planet is influenced by its rotation rate and internal composition. Faster rotation rates tend to cause greater flattening, resulting in higher ellipticity values. Planets with more massive and rigid cores, surrounded by less dense and deformable mantles, may also exhibit higher ellipticity due to the redistribution of mass caused by gravitational forces.