The Federation Reserve Fleet was a battlegroup of starships that existed within Starfleet.

In the year 2377, when the Federation sent its main forces through the Borg transwarp portal, they left the Reserve Fleet behind along with their Klingon allies to guard the Alpha Quadrant. The Reserve Fleet, at its headquarters in the Pearl Nebula, suffered from a surprise assault from the Cardassian Union who devastated the Starfleet forces.

Had it not been for the assistance of the Klingon Empire, the Federation would have suffered high casualities from the Cardassians surprise attack. (TNG video game: Armada II)