A Stack is a scaling effect or buff that can be increased through specific means or actions. Stacks are most often applied through dealing damage, where one instance applies one stack. Most stacking effects are built up quickly and are only temporary, disappearing all at once after their duration runs out. Additionally, there are stacking bonuses that apply directly to heroes, making them stronger, with a few of these bonuses sometimes being permanent. Only a handful of heroes can infinitely stack, allowing them to constantly scale as a game goes on.
While most stacking items effects are temporary, Glass Cannon, Siphon Bullets and Restorative Locket are the only items to semi-permanently keep their stacks, and require certain conditions to lose them.
Mirage's bullets apply a mark, stacking upon subsequent shots and allowing Mirage to detonate the stacks remotely, damaging all enemies marked based on their stacks.
Heroes with Permanent Stack Bonuses
There are currently 3 Heroes capable of infinitely stacking during a round. In theory, they can continue to scale endlessly with no diminishing returns.