* Radial Velocity refers to the velocity of an object towards or away from the observer.

* Zero Radial Velocity means the object is neither moving towards nor away from the observer.

While a zero radial velocity can occur in elliptical or parabolic orbits, it is not a defining characteristic of these orbits.

Here's why:

* Elliptical Orbits: In an elliptical orbit, the object's speed varies as it travels around the central body. There are points in the orbit where the radial velocity is zero. This occurs at the apoapsis (farthest point from the central body) and the periapsis (closest point to the central body).

* Parabolic Orbits: A parabolic orbit represents an escape trajectory. The object's speed is constant along the parabolic path, so its radial velocity is never zero.

* Circular Orbits: A circular orbit has a constant radial velocity of zero, as the object maintains a consistent distance from the central body.

In conclusion: A zero radial velocity is a specific point in an elliptical orbit but doesn't define it. It's also possible for a circular orbit to have zero radial velocity at all times.