The Byzantium was a category four Galaxy class Starliner.
History
The Eleventh Doctor, River Song and Amy Pond witnessed the Byzantium crash during the 51st century. It carried a Weeping Angel and when the ship crashed, the Angel was let loose. (TV: The Time of Angels) It transpired that the Angel had sabotaged the ship and forced it to crash, so its starving brethren on Alfava Metraxis could feed off the ship's energy. Once crashed, the ship began to emit radiation from its drives, harmful to humans but a source of food for the Weeping Angels.
The Byzantium could travel for ten years between planetfalls, partially due to it having an enormous forest within itself, with modified trees to convert starlight into oxygen for the crew to breathe. (TV: Flesh and Stone)
River Song mentioned to Amy Pond that she just returned from the Byzantium when telling her that the Doctor didn't really die and that she was married to him. (TV: The Wedding of River Song)
When River Song met the Tenth Doctor just before her death, she mentioned the crash of the Byzantium from her diary. (TV: Silence in the Library)
