Orbital element

An orbital element is one of a set of parameters specifying an orbit at a chosen epoch. For a Kepler orbit, the semi-major axis and eccentricity determine its size and shape, while angular elements determine its orientation and the body's position.

The true anomaly is the angle at the focus from periapsis to the current position of an orbiting body, measured in the direction of motion.

Orbital inclination is the angle between an orbital plane and a reference plane.

The longitude of periapsis gives the angular direction of periapsis in the reference plane.

Mean longitude is an angle that advances uniformly at the mean motion in an unperturbed Kepler orbit.

Mean motion is the average orbital angular frequency. A Kepler orbit about gravitational parameter $\mu$ has $n^2{a}^3=\mu$.