Galactic Plane
The Galactic Plane is the plane in which the majority of a disk-shaped galaxy's mass lies. In the case of the Milky Way, it is the plane of the galaxy's disk, encompassing the spiral arms, stars, gas, and dust. This plane is used as the reference for the galactic coordinate system.
Source: asd.gsfc.nasa.gov
APODs including "Galactic Plane"
GLIMPSE the Milky Way
16/12/2005
Scroll right and gaze through the dusty plane of our Milky Way Galaxy in infrared light. The cosmic panorama is courtesy of the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) project and the Spitzer Space Telescope. The galactic plane itself runs through the middle of the false-color view that spans nine degrees (about 18 full moons) across the southern constellation Norma. Spitzer's infrared cameras see through much of the galaxy's obscuring dust revealing many new star clusters as well as star forming regions (bright white splotches) and hot interstellar hydrogen gas (greenish wisps). The pervasive red clouds are emission from dust and organic molecules, pocked with holes and bubbles blown by energetic outflows from massive stars. Intensely dark patches are regions of dust too dense for even Spitzer's infrared vision to penetrate.