A sight is a device used to assist aligning or aiming weapons, surveying instruments, or other items by eye. They can also be optical devices that allow the user to see the image of an aligned aiming point in the same focus as the target. Iron sights are normally the default sight built into all firearms.
In all Battlefield games since Battlefield: Bad Company 2, the player can unlock and equip optical sights in place of the default weapon iron sights. These optical sights range from red dot sights to magnified rifle scopes and serve to give the shooter a better view while in ADS and aid in target acquisition. Additionally, some sights, such as high zoom rifle scopes, can only be attached to certain weapons.
In most Battlefield games, vehicles have access to unlockable imaging/target acquisition systems. Commonly, these vehicle specializations include some sort of thermal imaging upgrade or enhanced zoom upgrade to the vehicle's main sights. In games where the player can choose their vehicle loadouts, optical upgrades are often available to many different vehicle types (in some cases however, the same upgrade can have slightly different effects depending on the vehicle it is equipped to).