Galactic Plane

the plane which passes through the centre of the MILKY WAY, as far as that can be determined.

Taken from Dictionary of Science

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The plane in which the disk of the Milky Way Galaxy lies. By definition, one direction perpendicular to this plane is called above or north, and the opposite direction, also perpendicular to the galactic plane, is called below or south. From Earth, due galactic north is marked by the north galactic pole, which lies in Coma Berenices, near the bright star Arcturus, and due galactic south is marked by the south galactic pole, which lies in Sculptor.

Taken from The Internet ENCYCLOPEDIA OF SCIENCE