Johannes Kepler invented "anomalies" (angles) originally used to define an planet's orbit motion around the Sun.

Mean Anomaly (M): The angle measured since perigee that would be swept out by the satellite if its orbit were perfectly circular. This hypothetical orbit would assume the real orbit's semi-major axis and its period. The Mean Anomaly indicates where the satellite was in its orbit at a specific time.

Eccentric Anomaly (E): The angle, measured since perigee, based on the hypothetical position on the circular orbit defined by a line perpendicular to the major axis that passes through the true position of the satellite and intersects with the circular orbit. The Mean Anomaly is directly related to the Eccentric Anomaly through Kepler's Equation.

True Anomaly (n): The true angle, measured since perigee, that the satellite sweeps out while orbiting the Earth.