(1) The angle from the line of apsides of an elliptical orbit to the radius vector from the attracting focus to a point moving on the ellipse with angular velocity equal to the average velocity of the actual, moving body. It is related to the eccentric anomaly by Kepler's equation. It is related to the period P of the body and to the time t since the body passed through pericenter by the formula (M) = 2πt/P.

(2) In undisturbed elliptical motion, the product of the average angular speed times the interval of time since the body passed pericenter center. Thus the mean anomaly is the angle from pericenter to a hypothetical body moving with a constant angular speed which is equal to average angular speed.