The thick disk is one of the structural components of several galaxies, including the Milky Way. It was first proposed as a unique galactic structure, different from the thin disk and the halo in an 1983 article by Gilmore & Reid. It is supposed to dominate the stellar number density between 1 to 5 kiloparsecs (3.3 to 16.3 kly) above the Galactic plane and is composed almost exclusively of older stars. Its chemical composition and kinematics (those of the stars comprising it) are also said to set it apart from the thin disk. Stars within the thick disk may be called old-disk stars. Compared to stars in the thin disk, these typically have a significantly lower levels of metals—that is, the abundance of elements other than hydrogen and helium.
The thick disk is a source of early kinematic and chemical evidence for a Galaxy's composition and thus is regarded as a very significant component for understanding Galaxy formation.
Various scenarios for the formation of this structure have been proposed, including:
- Thick disks come from the heating of the thin disk.
- More energetic stars migrate outwards from the inner galaxy to form a thick disk at larger radii.
- It is a result of a merger event between the Milky Way and another dwarf galaxy.
- Multiple small galactic mergers disturb stars from the thin disk so that progressively older stars are scattered out further, and even more so, far from the galactic center.
- The disc forms thick at high redshift with the thin disc forming later 
Although the thick disk is mentioned as a bona fide galactic structure in numerous scientific studies and it's even thought to be a common component of disk galaxies in general, its nature is still under dispute.
The view of the thick disc as a single separate component has been questioned by a series of papers that describe the Galactic disc with a continuous spectrum of components with different thicknesses.
- Thin Disk
- Galaxy formation and evolution
- Galactic coordinate system
- Galactic bulge
- Disc galaxy
- Spiral arm
- Galactic halo
- Gilmore, G.; Reid, N. (1983). "New light on faint stars. III - Galactic structure towards the South Pole and the Galactic thick disc". Monthly Notices of the Royal Astronomical Society. 202 (4): 1025. Bibcode:1983MNRAS.202.1025G. doi:10.1093/mnras/202.4.1025.
- Bensby, T.; Feltzing, F. (2009). "The Galactic thin and thick discs in the context of galaxy formation". Proceedings of the IAU Symposium. 265: 300. arXiv:. Bibcode:2010IAUS..265..300B. doi:10.1017/S1743921310000773.
- Kordopatis, G.; et al. (2011). "A spectroscopic survey of thick disc stars outside the solar neighbourhood". Astronomy & Astrophysics. 535: A107. arXiv:. Bibcode:2011A&A...535A.107K. doi:10.1051/0004-6361/201117373.
- Freeman, K. C. (2010). "The HERMES Project: Reconstructing Galaxy Formation". In Block, D. L.; Freeman, K. C.; Puerari, I. Galaxies and their Masks: A Conference in Honour of K.C. Freeman, FRS. Springer. p. 319. Bibcode:2010gama.conf..319F. doi:10.1007/978-1-4419-7317-7_27. ISBN 978-1-4419-7316-0.
- Steinmetz, M. (2012). "The Galactic thin and thick disk". Astronomische Nachrichten. 333 (5-6): 523. arXiv:. Bibcode:2012AN....333..523S. doi:10.1002/asna.201211698.
- Schoenrich, R.; Binney, J. (2009). "Chemical Evolution with Radial Migration" (PDF). Monthly Notices of the Royal Astronomical Society. 396 (1): 203–222. arXiv:. Bibcode:2009MNRAS.396..203S. doi:10.1111/j.1365-2966.2009.14750.x.
- Loebman, S.; et al. (2011). "The Genesis of the Milky Way's Thick Disk via Stellar Migration". The Astrophysical Journal. 737 (1): 8. arXiv:. Bibcode:2011ApJ...737....8L. doi:10.1088/0004-637X/737/1/8.
- Fohlmeister, J. (24 April 2015). "The riddle of galactic thin–thick disk solved". Phys.org. Retrieved 24 May 2015.
- Brook, C. B; Kawata, D.; Gibson, B. K.; Freeman, K. C. (2004). "The Emergence of the Thick Disk in a CDM Universe". The Astrophysical Journal. 612 (2): 894. arXiv:. Bibcode:2004ApJ...612..894B. doi:10.1086/422709.
- Yoachim, P.; Dalcanton, J. (2012). "Structural Parameters of Thin and Thick Disks in Edge-On Disk Galaxies". The Astronomical Journal. 131 (1): 226–249. arXiv:. Bibcode:2006AJ....131..226Y. doi:10.1086/497970.
- Schoenrich, R.; Binney, J. (2009). "Origin and Structure of the Galactic Disc(s)". Monthly Notices of the Royal Astronomical Society. 399 (3): 1145–1156. arXiv:. Bibcode:2009MNRAS.399.1145S. doi:10.1111/j.1365-2966.2009.15365.x.
- Bovy, J.; Rix, H.- W.; Hogg, D. W. (2012). "The Milky Way Has No Distinct Thick Disk". The Astrophysical Journal. 751 (2): 131. arXiv:. Bibcode:2012ApJ...751..131B. doi:10.1088/0004-637X/751/2/131.