Constellation

A constellation was a group of stars.

Definition
Time Lords defined constellations as a group of stars based on their temporal and physical locations. They also included stars that hadn't been created yet and stars that had died out. This concept was later used in humanity's distant future. The Kagananaga Botizoids had a similar concept, but had a different name for it. (PROSE: The Devil Goblins from Neptune)

Individual constellations
Kasterborous was the location of the planet Gallifrey. (TV: Pyramids of Mars)

Scytha was the location of the planet Ribos. (TV: The Ribos Operation)

Cetus was the location of the planet Varos. (TV: Vengeance of Varos)

Fornax, Signus A, Omega Centauri, and Andromeda were constellations whose planets fell to the Committee. (AUDIO: The Conspiracy)

In a parallel world, the stars in the Orion constellation burnt out as a result of "the Darkness", (TV: Turn Left) also known as the reality bomb, which was detonated on the Crucible in the Doctor's world. (TV: Journey's End)

The Orpheus constellation was located ten thousand light years away from Earth. (TV: Aeolian)

At the end of the 22nd century Dalek invasion, the First Doctor warned that "the entire constellation" would be upset had the Daleks succeeded in their plot to pilot the Earth out of its orbit. (TV: The Dalek Invasion of Earth)

Miros was located millions of light-years from Mutter's Spiral. (TV: Mission to the Unknown)

Other references
Purporting himself to be a "Doctor Vile" to Earth Command, the Tenth Doctor called himself "Pilot of the Constellations". (TV: The Infinite Quest)

In a vain plea to convince to leave the Earth, the Tenth Doctor offered to fight him "across the constellations". (TV: The Sound of Drums)

In the Constant Division in the 67th century, "the constellations" were invoked in an incantation led by Ronan in memory of Eve Cicero. (TV: The Tsuranga Conundrum) "May the saints of all the stars and constellations. Bring you hope as they guide you out of the dark and into the light on this voyage and the next, and all the journeys still to come."

- Ronan

Behind the scenes
As acknowledged by The Discontinuity Guide, constellations in the real world are arbitrary divisions of stars in a planet's sky. As such, the Doctor's references to "the constellation of Kasterborous" would only make sense if Kasterborous was a constellation in a particular planet's sky. The Discontinuity Guide speculates that the planet in question in Earth, bearing in mind the Doctor's familiarity with it, as well as citing Marc Cory's reference of the constellation of Miros, which does not exist in the real world, in TV: Mission to the Unknown. Furthermore, it is suggested that at some point between the years 1995 and 3999, the constellations of Earth were renamed to include Kasterborous, Miros ("Mir"), Canthares. (TV: Image of the Fendahl) Scytha ("Skytha") (TV: The Ribos Operation) and "Cetes (possibly Cetus)". (TV: Vengeance on Varos)