Air Superiority is a tactic that has been used, or at least attempted since the Great World War II. It refers to having complete domination of the skies over the battlefield, and therefore a highly important strategic advantage.
Air superiority does not guarantee victory on the battlefield. It does however increase the chances of the commander that is using the tactic to gain victory over his enemies.
It was first attained by the USSR, which gained air superiority with their Yak-9 and MiG-29 fighters after ground forces took out Allied AAA (Antiair Artillery) positions.
In the First Tiberium War both sides were quite evenly matched in terms of planes. GDI possessed Carrier borne F/A-18E/F Super Hornets and Advanced YF-23 Black Widows (as well as the ORCA, which was not deployed until near the end of the war,) while Nod used RAH-66 Comanches and the only fighter they have are the Advanced F-22 Raptors. Both sides developed powerful anti-aircraft defences, such as SAM sites.
During the Second Tiberium War Nod gained the upper hand with its technologically advanced Banshee fighter and Harpy helicopters, but GDI's powerful ORCA Fighters and Orca Bombers remained a threat. Aircraft proved very useful against the Core Defender, at least during the Firestorm Crisis.
During the Third Tiberium War, Nod relied heavily on its versatile Venom aircraft, while GDI deployed the Firehawk, a fairly traditional fighter-bomber which causes great amounts of damage each sortie, but can only be loaded-out to engage either aircraft or ground targets, not both at once. The Firehawk is notable for the Stratofighter ability, which allows it to climb to the stratosphere (far beyond the normal range of any antiair defense or most fighters) then descend near the targeted area- jumping over any troublesome AA defenses in the area, though unless micromanaged they will run into said defenses on the way back, and not likely survive. Nod Vertigo stealth bombers appear seemingly from nowhere (having technology related to the Stealth Tank) to drop single, powerful Fuel-Air Bombs on a target, and while in flight, any aircraft behind a Vertigo are vulnerable to its tail-mounted antiaircraft gun turret. GDI uses Orca Gunships, which are useful airborne scouts (though, unlike their Nod counterpart, the Venom, they must reload and cannot engage air units) due to their Sensor Pods that can temporarily observe an area (or a unit, should that have been the target) and reveal stealth, while the ORCA itself detects stealth with Sensor Scans. However, both Nod and GDI were outgunned in the skies by the Scrin, whose Spaceships like Planetary Assault Carriers could see off multiple human aircraft. Devastator Warships and Stormriders also destroy the GDI AA Guns and Nod SAMs to maintain control over the sky.
In fact, the Scrin stand out so much in the skies they deserve special mention. The Scrin almost need at least local air superiority to attain victory. Their analog to human MCVs, the Drone Ship, flies. Their two most powerful non-Epic units, the Planetary Assault Carrier and Devastator, fly. Their city-obliterating Mothership flies. The Mothership is the most powerful air unit (per hit) potentially in the entirety of all three Tiberium Wars AND all the various Great World Wars. Its Catalyst Cannon, if it does not destroy a target immediately, causes it to explode a few moments later. This delayed explosion's shockwave causes the same effect on everything it hits. It can, therefore, if it hits the right place, destroy an entire base in just one shot. (It has the disadvantage of this being an unranged attack, costing 5000 credits, moving even slower than a Kirov Airship of GWWIII fame, and being 100% unarmed against aircraft, not to mention that a single Scrin player may only ever have one Mothership at a time.)The Devastator is a flying artillery piece, comparable in firepower to two Juggernauts, if lacking the bombardment capabilities. It outranges all air defenses, even the Scrin's own Storm Column, and can be a major threat to an otherwise well-defended base. The Planetary Assault Carrier carries eight small, powerful fighters that engage both surface and airborne targets, firing weak bursts of energy many times, returning, then repeating the process. All of the Scrin aircraft improve in Ion Storms and the PAC can also, when stationary, generate an Ion Storm, much like a Storm Column can. Additionally, PACs and Devastators can have Force Shields added (which even stop a single EMP blast from instant-destroying them. This is the other weakness of a Mothership.)
In the Final War both GDI and Nod must deploy support Crawlers to maintain an air force.