An assault fleet is a group of starships placed together to combat the enemy.
The Terran Empire formed assault fleets in the mid-22nd century in order to combat the rebellion that had emerged at the time, and had defeated the Empire in several battles, including the one at Tau Ceti.
ISS Enterprise and ISS Avenger were among the group of Starfleet combat ships that had been assigned to an assault fleet. While Enterprise never joined the fleet, Avenger had and was the sole survivor of the fleet after it had engaged rebel starships. (ENT: "In a Mirror, Darkly")