Wiktionary
n. (context astronomy celestial mechanics English) An angle of (radians) divided by the orbital period (of a celestial body in an elliptic orbit).
Wikipedia
In orbital mechanics, mean motion (represented by n) is the angular speed required for a body to complete one orbit, assuming constant speed in a circular orbit which completes in the same time as the variable speed, elliptical orbit of the actual body. The concept applies equally well to a small body revolving about a large, massive primary body or to two relatively same-sized bodies revolving about a common center of mass. While nominally a mean, and theoretically so in the case of two-body motion, in practice the mean motion is not typically an average over time for the orbits of real bodies, which only approximate the two-body assumption. It is rather the instantaneous value which satisfies the above conditions as calculated from the current gravitational and geometric circumstances of the body's constantly-changing, perturbed orbit.
Mean motion is used as an approximation of the actual orbital speed in making an initial calculation of the body's position in its orbit, for instance, from a set of orbital elements. This mean position is refined by Kepler's equation to produce the true position.
Usage examples of "mean motion".
After having considered for a few minutes, a soft bunch with a black, open eye, which perhaps was looking at them, perhaps was not, it hobbled calmly forward and began to nibble the grass with that mean motion of a rabbit's quick eating.