The term stack in World of Warcraft generally refers to three different things:

  1. A grouped number of the same items which take up only one bag or bank slot.
  2. The ability for multiple buffs or debuffs to remain active at the same time. If they can work together, they can "stack". Some buffs and debuffs can be applied multiple times to stack with themselves.
  3. Characters are "stacked" when they are so close together in the game world that they overlap.

Relating to Items & Inventory

  • 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

  • "Stacking" the buffs from two abilities can greatly enhance your combat effectiveness. For instance, one ability that enhances your attack speed and another that enhances your critical strike chance results in more attacks with the higher critical strike chance, which is better than using the two abilities separately.
  • Most spells and effects that are similar or identical to each other, especially debuffs, 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% (the maximum of the two effects), not 100%.
  • Some spells and abilities can apply an ongoing effect multiple times, thus "stacking up" the effect and increasing its power. Examples include [Sunder Armor],  [Vengeance], and [Arcane Blast].