From Halopedia, the Halo wiki
Battlegroups are subdivisions of a UNSC fleet, flexible fighting formations that can formed on an as-needed basis in several different tactical formations, which themselves can be "plugged together" into ad-hoc task forces. Battle groups are usually created on a temporary basis, with a specific mission or to accomplish a specific task. Battle groups can range in size from as little as three ships to larger carrier groups with smaller "tactical" carriers at their core. These battle groups are referred to as carrier groups.
Under the NAVCOM rank structure, battle groups are usually commanded by a rear admiral, though there have been instances of smaller battle groups being led by captains. Each battle group is assigned a number or a code name.
The concept of a flight may be related to the concept of battle groups.
Known UNSC battle groups
Although the Covenant organize smaller fleet units on a temporary basis similar to UNSC battle groups, their naval doctrine identifies such units as fleets rather than battle groups; for example, the Fleet of Sacred Consecration was still identified as a fleet despite only consisting of 15 ships. Instead, the term "battle group" is applied to certain tactical formations of Covenant ships that are part of a larger fleet; for example, the Fleet of Particular Justice had several "CCS-class battle groups", each consisting of three CCS-class battlecruisers. Similar to the naming of their ships and fleets, the Covenant themselves often derived the naming of their battle groups from their religious nomenclature.