The term stack in World of Warcraft generally refers to two different things:
- A grouped number of the same items which take up only one bag, bank, or equip slot.
- The ability of ability, buff, debuff, power, or spell effects to remain active at the same time. If they can work together, they can "stack".
Both have numbers in the lower right corner to indicate how many times they have "stacked".
Relating to Items & Inventory Edit
- Many items, particulary reagents, can be "stacked" in your inventory. This just means that more than one of an item can be represented by a single icon in a single slot in your inventory. The icon will then have a number appear in the lower-right corner to show how many of that item you have.
- Items can be divided/unstacked by SHIFT+CLICKing the icon in the inventory.
- Items can also be restacked by dragging one icon onto the other. If they can't stack, they will simply swap places in your inventory.
- Stack sizes vary depending on the item. Common sizes are 5, 10, 20, and 200.
- Many items, particularly large items, can't be stacked — items such as weapons, armor, and shields, for example.
Relating to Spells & Effects Edit
- "Stacking" spells can greatly enhance your character, but most spells and effects that are similar or identical to each other cannot affect you at the same time, so it is said that the spells don't stack. For example, if you are affected by both Wing Clip and Hamstring, your movement speed will be reduced by 50%, not 100%.
- Some spells and abilities can apply an effect to a target multiple times, thus "stacking up" the effect and increasing its power. Examples include Deadly Poison, Sunder Armor, Vengeance, and Arcane Blast