The angle between the periapsis of an orbit and a given point on a circle around the orbit, as seen from the centre of the orbit (see diagram). The point concerned is found by drawing a line perpendicular to the major axis through the actual position of the orbiting body until it cuts the circle around the orbit, as in the diagram. The diameter of the circle is equal to the major axis of the orbit, and its centre is also the orbit’s centre. The angle of eccentric anomaly is measured in the direction of orbital motion. See also Kepler’s Equation.

eccentric anomaly:

The eccentric anomaly of a planet’s position is the angle ACQ, where A is the perihelion, C is the centre of a circle drawn around the planet’s elliptical orbit, and Q is a position on that circle found by drawing a line perpendicular to the major axis of the orbit through the actual position of the planet, P. The Sun is at S.