An Ambassador-class Federation starship that was undergoing repairs in 2368 when William Riker took temporary command of it for Picard's blockade during the Klingon Civil War.
The USS Excalibur was an Ambassador-class starship of the 24th century, named like its Constitution-class predecessor after King Arthur's sword.
In Redemption II, Captain Picard has to assemble a fleet at short notice to stop the Romulans supplying the Duras family, and several ships are taken straight from the yards with borrowed officers. Riker is given the Excalibur, with Geordi La Forge as his first officer, while Data takes the Sutherland.
The Excalibur holds a place in the tachyon detection grid. When Picard later orders the fleet to fall back and re-form, Riker's ship is among those that move off, a redeployment Data ignores when he sees a chance to expose the cloaked Romulan convoy.