An Excelsior-class Federation starship that carried the propulsion specialist Kosinski and his assistant to the Enterprise-D in 2364 after apparently successful warp drive upgrades.
The USS Fearless was an Excelsior-class starship. It appears at the start of Where No One Has Gone Before, the first-season episode of The Next Generation that introduces the Traveler.
The Starfleet engineer Kosinski had run a series of adjustments on the warp drives of the Fearless and the Ajax, and both ships reported improved performance. The Fearless meets the Enterprise-D to pass him on so the same work can be done on a Galaxy-class engine.
Riker and Data are sceptical, since Kosinski's equations seem meaningless. They are right: the gains came from his quiet assistant, the Traveler, whose abilities throw the Enterprise millions of light years off course.