Laplace plane [SEP] named after
Laplace plane
The Laplace plane or Laplacian plane of a planetary satellite, named after its discoverer Pierre-Simon Laplace (1749–1827), is a mean or reference plane about whose axis the instantaneous orbital plane of a satellite precesses. 
Laplace's name is sometimes applied to the invariable plane, which is the plane perpendicular to a system's mean angular momentum vector, but the two should not be confused. They are equivalent only in the case where all perturbers and resonances are far from the precessing body.
Definition.
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This plane is sometimes called the "Laplacian" or "Laplace plane" or the "invariable plane of Laplace", though it should not be confused with the Laplace plane, which is the plane about which orbital planes precess. Both derive from the work of (and are at least sometimes named for) the French astronomer Pierre Simon Laplace. The two are equivalent only in the case where all perturbers and resonances are far from the precessing body. The invariable plane is simply derived from the sum of angular momenta