Answer: Eccentricity
The orbit of a body around another in space, as is the case of the Earth orbiting the Sun, is described by six orbital elements that determine its orientation, position, size and shape.
In the specific case of the shape of the orbit, this is determined by its eccentricity, which varies between 0 and 1.
When the eccentricity is 0, the shape of the orbit is circular, when this value begins to vary until approaching 1 (without reaching 1), the shape of the orbit becomes more elliptical.
In this sense, the eccentricity of the orbit of the Earth around the Sun is 0.2, which means is nearly circular. However, due to orbital perturbations this value may change over time.