The Imperial Grand Fleet was the main Zentraedi armada headed by Dolza, consisting of roughly five-million ships capable of destroying entire planets. It also served as the mobile home of the majority of the Zentraedi species.
It's nearest known counterpart, the Imperial Glass Fleet, consisted of only one-million ships.
The existence of the Imperial Grand Fleet was first confirmed by Lisa Hayes, Rick Hunter, and Ben Dixon when they were captured by the Zentraedi. Upon arrival in space controlled by the Robotech Masters, the trio saw the fleet with their own eyes, and Dolza made a demonstration of it's firepower on a nearby planet. (Macross Saga: First Contact)
Dolza later lead the armada to Earth to destroy the planet. While he was successful in devastating Earth, the Grand Cannon decimated their ranks, and the SDF-1 and the Imperial Glass Fleet under the command of Breetai launched a successful counterattack destroyed the majority of the once mighty fleet, including Dolza's command station, ending the armada's reign of terror. (Macross Saga: Force of Arms)