A Federation starship of the 23rd century that was due to take vaccines from the Enterprise in 2268 and was later crippled by the Whale Probe, its crew rigging a solar sail to survive.
The USS Yorktown is mentioned in two eras. In the Original Series episode Obsession, the Enterprise is due to meet the Yorktown to hand over perishable vaccines urgently needed on Theta VII, a rendezvous Captain Kirk delays while he hunts the cloud creature that once killed half the crew of the Farragut.
In Star Trek IV: The Voyage Home, a ship of the same name is among those disabled by the alien probe in 2286. Its captain, Joel Randolph, reports to Starfleet that the chief engineer is trying to deploy a makeshift solar sail to keep the crew alive.
The ship itself is never seen clearly. The name was also the one Gene Roddenberry first chose for the Enterprise in his 1964 series outline.