Stack

Stacking can refer to a number of situations.

Inventory Stacking
The most obvious use of the term stacking is stacking items in your inventory. When you have a large number of items of the same type in your inventory they will generally all sit in the same slot with a number indicating how many of that item you possess. Some items, such as Salvage Items, are not stackable. If you have two salvage items in your inventory then they will not sit in the same slot.