Laplace–Runge–Lenz vector (Physics Dictionary)

Laplace–Runge–Lenz vector: Also abbreviated LRL vector.

A vector used chiefly to describe the shape and orientation of the orbit of one astronomical body around another, such as a planet revolving around a star. For two bodies interacting by Newtonian gravity, the LRL vector is a constant of motion, meaning that it is the same no matter where it is calculated on the orbit; equivalently, the LRL vector is said to be conserved.