Aim down sights
From Destinypedia, the Destiny wiki
When aimed down sights, the camera angle shifts to behind the weapons sights. This generally makes the weapon more accurate, and may provide some zoom for more usability at longer ranges. Unlike most other instances of aiming down sights, weapons in the game are sufficiently accurate when the sights are not in use, and movement does not appear to be impeded when they are in use either. As of yet, it is unknown if the aim snaps onto enemies when entering ADS. However, what is certain is that the sights of a weapon are designed to "lock on" to targets because of an in-game mechanic called aim assistance. Aim assistance is the amount that the sights of your equipped weapon "stick" to a target. The higher the aim assistance, the more it sticks, and so it becomes easier to focus on that target. This makes it especially helpful when tracking and gunning down fast-moving enemies. Aiming down sights will also make the radar invisible.
Some weapons in Destiny have bonus accuracy during hip-fire, making it less necessary to aim down their sights.
The Viper P3 when aiming down sights.