The galactic plane is the plane where the disk of the Milky Way Galaxy lies.
Any object located perpendicularily above this plane is descibed as being X° (degrees) North, and likewise, any object located perpendicularily below this plane is described as being X° South.
While approaching the Questar M-17 in 2269, the Enterprise experienced severe hypergravitional effects from the dead star's negative star mass, pulling it "two minutes right ascension" off course. As they entered orbit of the star Uhura detected a mysterious radio signal coming from an insectoid pod ship located at "nine seconds right ascension from galactic plane, dead ahead" of the Enterprise position in orbit of the star. (TAS: "Beyond the Farthest Star")