A galaxy was a dense grouping of billions of stars and other matter which was held together by powerful gravitational attraction.
Most of the Doctor's adventures took place in the galaxy known to the Time Lords as Mutter's Spiral, although he was also known to have visited other galaxies such as the Andromeda Galaxy and the Isop Galaxy. (PROSE: Alien Bodies, TV: Castrovalva, The Web Planet)
The Daleks recreated the Seriphia Galaxy with the Apocalypse Element and used it as their main base of operations during their invasions of Mutter's Spiral. (AUDIO: The Apocalypse Element, Dalek Empire)
The Tiberian spiral galaxy was a galaxy sacrificed by the human empire to destroy all Cybermen, bringing the Cyber-Wars to an end. (TV: Nightmare in Silver)
Whilst travelling with the Tenth Doctor, Donna Noble witnessed the destruction of the Mestophelix Galaxy. (COMIC: The Widow's Curse)
The Moment was known as the "Galaxy Eater". (TV: The Day of the Doctor)