Galactic plane

The galactic plane is the plane in which most of a disk-shaped galaxy's mass lies. 
The directions perpendicular to the galactic plane point to the galactic poles. In actual use, the terms "galactic plane" and "galactic poles" usually refer specifically to the plane and poles of the Milky Way, the galaxy where the Earth is.
Some galaxies are irregular and do not have any well-defined disk. Even in the case of a spiral galaxy like the Milky Way, defining the galactic plane is imprecise since the stars are not perfectly in the plane.