Reticule bloom
From Destinypedia, the Destiny wiki
Bloom, or reticle bloom, is a mechanic in video-games. This is when the reticle on the hud becomes larger with sustained fire, making the weapon less accurate. The reticle will return to normal after a sufficient time has passed without firing.
This mechanic is found in most first-person shooters, such as Battlefield, Halo, and Call of Duty. As seen in the E3 2013 Destiny Gamplay Demo, there will be bloom in Destiny as well, though it appears that not every weapon will have it.
