A technologically ambitious starship designed with a transwarp drive that never fully worked.
The USS Excelsior was the prototype of the Excelsior class, introduced in Star Trek III: The Search for Spock as Starfleet's newest ship. It was fitted with an experimental transwarp drive and was commanded at the time by Captain Styles.
When Kirk and his officers stole the Enterprise from Spacedock, the Excelsior was ordered to chase them. Scotty had already sabotaged its transwarp system, and the new ship lost power as soon as it tried to go to warp, leaving the Enterprise to escape. The transwarp project was later abandoned, but the Excelsior class went on to become one of Starfleet's most common designs for decades.
In Star Trek VI: The Undiscovered Country, Captain Hikaru Sulu commanded the Excelsior on a survey mission when it was hit by the shock wave from the explosion of the Klingon moon Praxis. Sulu later defied orders to help Kirk, and the Excelsior joined the Enterprise-A in defeating General Chang's cloaked Bird-of-Prey at Khitomer.