Stealth is a term used to define an aircraft's capability to "camouflage" itself from radar systems. The term can be used to refer to either radar stealth or optical stealth. In a broader sense, "stealth" is defined as "an aircraft with a weak or non-existent radar signature". A common example (and arguably the most famous) of stealth aircraft is the F-117A Nighthawk.
A way to make an aircraft stealth is its airframe design: an aircraft's shape reflects radio waves emitted by radars. This design, however, has the inherent flaw of forcing an aircraft to have an internal weapons bay to retain its stealth capability, which is rendered useless while the aircraft readies its ordnance. Another way of making an stealth aircraft is radar-absorbing/"iron ball" paint, which minimizes the chances of radar detection by absorbing its waves. This last design option, however, is not failproof, as evidenced by the case of an F-117 shot down during the Kosovo War in 1999, which was detected by a long-wavelength radar while delivering payload.
The Ace Combat games allow both players and NPCs to employ stealth capabilities, with varying degrees of success. Under the player's use, stealth either renders the player invisible to the enemy unless at close range, or simply affects the enemy's missile lock capabilities. While used by the enemy, however, the aircraft will periodically disappear from radar for a short time period, forcing the player to visually pinpoint their location to engage.
In Air Combat, the F-117A Nighthawk is the first stealth aircraft (and indeed, the first aircraft) unlocked by the player, as well as the most common stealth aircraft used by the coup d'état forces. Other aircraft with stealth capabilities are the YF-23A Black Widow II, the YF-22 Lightning II and the B-2A Spirit.
In Ace Combat 6: Fires of Liberation, the CFA-44 Nosferatu in "The Liberation of Gracemeria" is able to disappear from radar and momentarily disengage, leaving the player to deal with the UAV-45 drones and the enemy's ADMM missile barrages. The F-14D -HALLOWEEN- DLC aircraft is also stealth-capable.
In Ace Combat X: Skies of Deception, the Gleipnir and the Fenrir employ a unique type of visual stealth, called the "Digital Optical Stealth, which renders them invisible to both radar systems and to the naked eye. The "CFRP Conversion", "RAM Coat" and "Diffusion Coat" Armor upgrades give the player's aircraft minor stealth capabilities.