Which orbital element is responsible for defining the *shape* of the path, specifically how much it deviates from a circle?

Answer

Eccentricity

Eccentricity is the specific orbital element that quantifies the deviation of the orbit from perfect circularity. The semi-major axis, conversely, determines the size of the orbit.

Which orbital element is responsible for defining the *shape* of the path, specifically how much it deviates from a circle?
Spaceastronomyorbitplanetpath