Stun is a Combat move in the Batman: Arkham video game series. Like the name suggests, a stun does not inflict damage onto an enemy, but merely renders them disoriented and open for another attack.
The stun was introduced in Batman: Arkham Asylum as one of the basic combat moves, along with the strike and Counter. To execute it, tap the stun button (Circle on PlayStation, B on Xbox and the mouse wheel on PC) while moving toward an enemy. By doing so, the character will stun the target enemies, making them open for a strike. The stun range is quite wide, approximately 180 degrees, making it possible to stun multiple enemies at once.
The stun is first introduced in Story Mode when a High Security Henchman comes out of one of The Joker's "present boxes". High Security Henchmen are quite dangerous as they deal a lot of damage and can block head-on strikes if they're fully on their feet. The only way to land a hit on them is to stun them first.
As Batman is the first and primary playable character, it's also called the Cape Stun, because Batman stuns enemies by swiping his cape right in front of them. Joker, on the other hand, stuns enemies by releasing laughing gas as he swings his arm in front of them.
A stun does not inflict damage or gives points, but adds 1 Combat variation.
Starting from Batman: Arkham City, knife users can no longer block attacks and can simply be hit by normal strikes. However, new types of enemies were introduced, including Armored Henchmen, Shield Users and Titans, along with new ways to use the stun.
With the introduction of armored enemies also came the Beat Down. Like the High Security Henchmen in Asylum, armored enemies can block head-on strikes while they're fully on their feet, so the only way to land a strike on them is to stun them first, then do a Beatdown on them until they're knocked out. The Beatdown can also be used on other types of enemies (except stun baton users and shield users). When doing a Beatdown on a stunned enemy, the character will repeatedly hit on the enemy's ribcage until they're finished.
Shield users in City can also block head-on strikes while they're fully on their feet, so the only way to land a strike on them is to do an Aerial Attack. To do it, stun the shield user, then double tap the "evade" button (Cross on PlayStation, A on Xbox and Space on PC). The character will climb over the shield and slam the enemy onto the ground. Due to these new uses of the stun, targeting armored and shielded enemies is prioritized.
Unlike in Asylum, Titans are no longer affected by a thrown Batarang, so the only way to hit them is to perform an Ultra Stun on them. An Ultra Stun requires three quick, consecutive stuns. A stunned Titan is rendered immobile for a while and open for a Beatdown. An Ultra Stun can also be used on other types of enemies, excluding shield users and lieutenants, and it will send them airborne until they're on the ground stunned for quite some time without inflicting any damage (a Ground Pound can be used as a follow-up). It can also be used on Ninjas, even though they can't be simply grounded conscious and they will get right back up. The Ultra Stun, however, is slow and leaves the character vulnerable, so it should only be used at the right time (when the crowd has been thinned enough) or on the right enemy (Titans).
Neither the stun or Ultra Stun gives any points, but each of them adds 1 combat variation.
From Batman: Arkham Origins onward, the Ultra Stun is called Super Stun.
Both the stun and Super Stun can be used in Batman: Arkham Knight. Armored enemies are now replaced by Brutes, who require more Beatdown strikes to finish.
In previous games, the Super/Ultra Stun could only knock an enemy to the ground and during the whole time the enemy's airborne there was nothing to be done. Therefore, a new move called Aerial Juggle was introduced. By immediately strike the airborne enemy (by kicking them, specifically), the character will send the enemy far away, knocking others in the way. This move is useful even against ninjas or Combat Experts, but can't be used on shielded enemies, Electrified henchmen and Brutes. The Aerial Juggle is not available for Batgirl.
Due to the slowness of the Super Stun, additional upgrades are available to speed it up. The Batclaw Super Stun can be used by Batman or Nightwing on an enemy who's staggering after being pulled by the Batclaw and it requires only 1 stun. The Charged Super Stun is also available for other characters, performed by "charging" 1 stun (holding the stun button a bit, then release).
The stun, the Super Stun, the Batclaw Super Stun and the Charged Super Stun each adds 1 combat variation.