A wing (also called an attack wing or unit) is a Starfleet designation for a grouping of between four and ten starships usually commanded by a senior captain. Multiple wings are usually grouped into a tactical wing.
There are three types of wings. If a wing is made up of only one class of starship, it is referred to as a class wing. This is the most common wing organization as ships of the same class work well together and resupply each other.
Finally, destroyer wings are mixed matched wings of vessels no larger than a cruiser.
Wings do not have a permanent designation as tactical wings do. If the Starfleet Fifth Fleet and Starfleet Second Fleet combine forces, for example, the 4th destroyer wing, Fifth Fleet might be changed to 12th destroyer wing as part of the combined force. (LUG RPG: The Dominion War Sourcebook: The Fires of Armageddon)
The Breen divided up their tilga'kars into wings called jelga. Made up of three to eight ships, jelgas usually contain vessels of the same size rather than mix capital ships with attack fighters. (The Dominion War Sourcebook: The Fires of Armageddon)
The Cardassians split their tar'elk into wings of 10 - 15 starships called a kel'elk (plural kel'elkor). A gul commanded a kel'elk from the lead vessel in the group. (The Dominion War Sourcebook: The Fires of Armageddon)
The Dominion also organized its forces into wings similar to Starfleet, generally in type wings. Wings made up of smaller vessels such as the Jem'Hadar attack ship were generally larger in number, with up to thirty vessels in a single wing. Wings made up of larger capital ships usually ranged between 3-10 vessels. A wing is usually commanded by a Vorta suba'toran. (The Dominion War Sourcebook: The Fires of Armageddon)
The Lyran Star Empire grouped ships into wings made up of a single type of hulls (although the flagships of wings often were more heavily armed and equipped than other ships in the wing) that could go from three ships, like Lion-class dreadnoughts to six ships for smaller ships like police cutters. They were commanded by commodores. (RIS Bouteina: "Broken Bread")
Star Trek: Banshee Squadron fighter wing
In Star Trek: Banshee Squadron, the term "wing" is used very loosely, inaccurately, and interchangeably with the term "squadron". A wing or squadron is simply a group of starfighters, usually numbering five, under the command of a Wing Commander.