What is the name of the ship hijacked by Khan in Star Trek II?

The ship hijacked by Khan in Star Trek II: The Wrath of Khan is the USS Reliant.

The Reliant is a Federation starship assigned to survey planets for the Genesis Project, a scientific experiment designed to transform barren worlds into habitable environments. While investigating the supposedly lifeless planet Ceti Alpha V, Captain Clark Terrell and Commander Pavel Chekov encounter Khan Noonien Singh and his exiled followers.

Khan seizes control of the Reliant and uses it to pursue Admiral James T. Kirk, whom he blames for his exile. The ship’s access to Genesis-related information makes it central to Khan’s plan. Viewers sometimes confuse it with the USS Enterprise because the two ships engage in the film’s major space battle, but the Enterprise remains Kirk’s vessel. The Reliant is ultimately destroyed in the Mutara Nebula after Khan activates the Genesis Device.