From Destinypedia, the Destiny wiki
Bloom, or reticule bloom, is a gameplay mechanic in many video-games. This is when the reticule on the heads-up display becomes larger with sustained or rapid fire, making the weapon less accurate. The reticule will return to its normal proportions after a sufficient time has passed without firing. Other actions, such as sprinting or walking can have an impact on bloom.
This game mechanic is found in many First-person shooters. As seen in the E3 2013 Destiny Gameplay Demo, bloom is also featured in Destiny, though it is not confirmed that every weapon will have it. 
List of appearances