Stealth is an ability available to the Operative, Assassin, Shadow and Scoundrel combat styles.
Stealth is also a combat mechanic. It is toggleable when outside of combat. While in stealth, most enemies cannot detect the player or their companions, unless they are extremely close to the player. Also, entering stealth slows the player by 15% and companions will not react to enemy attacks while in stealth. Attacking and most interactions break stealth and the player cannot reenter stealth for 5 seconds.