Answer: Eccentricity
From the 6 characteristic orbital elements of the orbit of a body around another more massive, its eccentricity will indicate the shape of the orbit.
Now, going to the specific case of planets, according to Kepler's 1st Law, the planets revolve around the Sun in elliptical orbits.
In this context, the ellipse is a conic, whose eccentricity
is between 0 and 1:
When its value is 0 we are talking about a circular orbit and when it is 1, a parabolic orbit.
That is, the nearer to the value of 1 (without reaching 1) the eccentricity of the orbit is, the more elliptical it will be.
Therefore, Planetary orbits that are more elliptical have a greater eccentricity.