Definitions for

**"Orbital elements"**define the shape, size and orientation of the path ("orbit") of an object around another object. The elements are the semi-major axis, eccentricity, inclination, argument of pericenter, longitude of the ascending node and time of perihelion (or alternatively, the mean anomaly and epoch).

The six numerical values that completely define the orbit of one body about another of known mass. They are the semi-major axis (a), the eccentricity (e), the inclination to the reference plane (i), the mean anomaly (M), the argument of the pericentre (), and the longitude of the ascending node (). The elements vary with time as a consequence of perturbations of other bodies, so their epoch is important. For comets and asteroids, the perihelion conditions are often of interest, so the date of perihelion (T) and perihelion distance (q) are usually used instead of M and a. (At T, M=0; q = a(1-e) )

Six quantities used to mathematically describe an orbit; i.e. semi-major axis, eccentricity, inclination, argument of periapsis, time of periapsis passage and longitude of ascending node.

A collection of quantities that, together, describe the size, shape, and orientation of an orbit.