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.