Gabriel Lorca's previous command, lost during the Klingon War.
The USS Buran was the Starfleet ship commanded by Gabriel Lorca before he took command of the Discovery during the Klingon War. It was lost in combat with Klingon forces.
Lorca's account was that he had destroyed the Buran himself with his crew still aboard, rather than allow them to be captured and tortured by the Klingons, and the story followed him through Starfleet. Later revelations about Lorca's true origin in the Mirror Universe cast doubt on how much of what he said about the ship and its loss could be trusted.