The Byzantium was a starliner which the Doctor witnessed crashing alongside River Song and Amy Pond. It carried the last of the Weeping Angels and when the ship crashed, the Angel was let loose (DW: The Time of Angels). When River Song goes through her diary in DW: Silence in the Library to check wherabouts the Doctor is in his timeline, she mentions "the crash of the Byzantium".
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The crashed Byzantium ship. (DW: The Time of Angels)