In celestial mechanics, the mean anomaly is the fraction of an elliptical orbit's period that has elapsed since the orbiting body passed periapsis, expressed as an angle which can be used in calculating the position of that body in the classical two-body problem. Wikipedia

A measure of time in the study of orbital dynamics Wikipedia Disambiguation

One of the orbital elements used to specify the orbit of an object in space Wikidata

An orbital parameter of a celestial body (in an elliptic orbit); the mean motion multiplied by the time since the last passage through periapsis; the angle PCM (measured in radians) defined by P = the periapsis, C = the center of the elliptical orbit, and M = the mean planet. Wiktionary

Astronomy: orbital parameter. Wiktionary (translation)