The Laplace plane is defined as the mean plane occupied by the orbit of a satellite during a precession cycle. In other words, the plane normal to the orbital precession pole of the satellite.
It is used to describe the orbits of satellites of gas giants. Roughly, the Laplace plane is the primary's equatorial plane for those satellites orbiting close to the planet, and progressively warps or tilts towards the orbital plane of the planet for its outer satellites. However, in some cases, such as the outer satellites of Jupiter (Ananke, Pasiphaë and the others), the perturbations induced by Saturn are sufficiently important to make the Laplace plane deviate from the simple description given here.