A circular orbit is the orbit at a fixed distance around any point by an object rotating around a fixed axis.
Below we consider a circular orbit in astrodynamics or celestial mechanics under standard assumptions. Here the centripetal force is the gravitational force, and the axis mentioned above is the line through the center of the central mass perpendicular to the plane of motion.
In this case not only the distance, but also the speed, angular speed, potential and kinetic energy are constant. There is no periapsis or apoapsis. This orbit has no radial version.
Read more about Circular Orbit: Circular Acceleration, Velocity, Equation of Motion, Angular Speed and Orbital Period, Energy, Delta-v To Reach A Circular Orbit, Orbital Velocity in General Relativity
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