Eccentricity is a dimensionless parameter that describes the shape of an orbit. It is defined as the ratio of the distance between the focus and the center of the ellipse to the semi-major axis.

The semi-major axis is the average distance between an orbiting body and the body it is orbiting.

Inclination is the angle between the orbital plane of an object and the reference plane.

Argument of periapsis is the angle between the ascending node and the periapsis of an orbit.

True anomaly is the angle between the periapsis and the current position of an object in its orbit.

Mean anomaly is the angle between the periapsis and the current position of an object in its orbit, averaged over time.

Eccentric anomaly is the angle between the periapsis and the current position of an object in its orbit, as seen from the center of the ellipse.

Orbital period is the time it takes for an object to complete one orbit around another object.

Sidereal period is the time it takes for an object to return to the same position in the sky as seen from Earth.

Synodic period is the time it takes for an object to return to the same position relative to the Sun as seen from Earth.

Anomalistic period is the time it takes for an object to return to the same perigee.

Mean motion is the rate at which the argument of periapsis changes.

True motion is the rate at which the true anomaly changes.

Eccentric motion is the rate at which the eccentric anomaly changes.

Anomalistic motion is the rate at which the anomalistic period changes.