The Earth's orbit is not a perfect circle, it is slightly distorted into an egg- shape, which the astrophysicists define as an ellipse.  The amount of this distortion is called the eccentricity of the Earth's orbit and it is measured as a unit of ellipsicity, which indicates the amount of variation from a perfect circle.  The eccentricity of the Earth's orbit varies between nearly 0 (a perfect circle to almost 0.05, currently it is 0.01671123 and it is getting smaller.