The Byzantium was a category four, Galaxy Class, Star Liner which the Doctor witnessed crashing alongside River Song and Amy Pond during the 51st century. It carried the a Weeping Angel and when the ship crashed, the Angel was let loose (DW: The Time of Angels). As it transpired, 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.
The Byzantium was described by the Doctor as being able to travel for 10 years before the need for planetfall, partially due to the it having an enormous forest within itself, with modified trees to convert starlight into oxygen for the crew to breathe. (DW: Flesh and Stone).
When River Song in her time line met the Tenth Doctor just before her death, she mentioned the Crash of Byzantium. (DW: Silence in the Library)
