Eccentricity describes the “roundness” of an orbit. It is a ratio between the semi-minor axis and semi-major axis of an orbit.
A “perfect” circular orbit has an eccentricity value of 0. The more eccentric the orbit (as e↑), the less circular the orbit.
An orbit is a trajectory with an e value less than 1 because an orbit is a closed path around a central body.
A trajectory with an eccentricity greater than or equal to 1 is an open trajectory, and the “orbiting” body will constantly move away from the central body.
For orbits or closed trajectories: 0 ≤ e < 1
For open trajectories: e ≥ 1