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A galactic disk (or galactic disc) is the disk portion of a disk galaxy, i.e., not including the galactic halo, the latter of which includes dark matter and can include globular clusters, field stars, clouds, and even (within the dark matter halo) dwarf galaxies. The galactic disk includes most of the stars, thus comprising the visible portion of the galaxy, and most of the interstellar medium (ISM) (gas and dust). The edge between galactic disk and surroundings is not clear-cut, and cited dimensions of the disk are generally based upon a defined criteria regarding the exponential falloff of emitted electromagnetic radiation over directions across the galaxy's plane away from the center, and over the two (opposite) directions perpendicular to the galaxy's plane.
The term disk is also used for sets of stars within the galaxy occupying a disk-shaped region, but which are also distinguished by characteristics of the star such as age and metallicity. Such disks can intersect with another or fully contain another. Regarding the Milky Way, the terms thin disk and thick disk are used.
The phrase capitalized (Galactic Disk) generally refers to that of the Milky Way, as does the phrase with the definite article (the galactic disk) when context does not indicate another galaxy.