In Star Trek: The Final Frontier, what is the name of the starship stolen by Sybok?

In Star Trek: The Final Frontier, the starship stolen by Sybok is the USS Enterprise-A.

Sybok, Spock's half-brother, takes control of the Enterprise-A after recruiting several members of its crew during a mission to Nimbus III. He believes that a powerful being is imprisoned beyond the Great Barrier and wants the starship to reach that location. Captain James T. Kirk and the rest of the crew are forced to follow as Sybok pursues his religious quest.

The Enterprise-A is the second Federation starship in the films to carry the Enterprise name. It is assigned to Kirk at the end of Star Trek IV: The Voyage Home after the original Enterprise is decommissioned. The ship remains the main vessel for the next three original-cast films.

The ship is not the USS Enterprise-D, which belongs to the later Next Generation era. It is also not the Excelsior, the advanced starship associated with Captain Hikaru Sulu in Star Trek VI: The Undiscovered Country.