A Stack can refer to several different things, as both a noun and a verb. When used in-game or in an article, one should use context to determine what type of "Stack" is being mentioned. Different types of "Stacks" and "Stacking" are included below:
Primarily, when someone mentions a "Stack," they are speaking of one instance of an effect which can multiply itself several times, effectively "stacking up" it's effect. An example of this would be Kayle's Holy Fervor, which makes her autoattacks apply a debuff which lowers armor and magic resistance upon all targets hit, and can stack up to 5 times, meaning that repeated application of the effect will increase the intensity of it, up to 5 times the intensity of one casting. When a "Stack" of this sort is applied, the duration of the whole resets itself. In the case of Kayle's Holy Fervor, the debuff lasts for 5 second, but as long as the debuff is continually applied with repeated attacks, the target will not lose the initial stack. Instead, all of the stacks will remain in effect until a full 5 seconds passes without the effect being re-applied. After this time period has passed, all the stacks of the effect will wear off simultaneously. For this reason, if one finds oneself inflicted with multiple stacks of a harmful effect, waiting for the debuff timer to pass once will remove the entire stack of effects. In the same vein, to keep up a beneficial stack such as the passive effect from Monkey's Agility, one needs only refresh the fully-stacked effect once each time it reaches the end of the duration, or they risk losing all the stacks.
Items with Stacking Effects
Several items have passive effects which can be stacked up. When a player mentions that they will be picking up a "snowball item" or that they are "stacking up" a particular item, they mean that they are picking up one of these items, or working on building up its passive effects. Several of these "Stacking Items" lose some or all of their stacks when a champion dies, making them a risky choice for players who constantly find themselves in compromising situations. Examples of these types of snowball items include:
Other types of stacking items permanently gain stats and do not lose any stacks upon death, making them safer choices, but typically require a large amount of stacks to reach the items' full potential. Examples of these items include:
Still other items do not gain any long-term stacks which increase their permanent effectiveness, but instead place a stacking debuff upon their opponents, or gain short-term stacks that benefit a character in each particular engagement. When a player mentions a "stacking item," these are usually not what they mean, but they do have stacks associated with them. Examples of these items are:
Stacking Multiple Items
A final usage of the term "Stack" or "Stacking" is the act of purchasing multiple copies of the same item, or multiple items with a particular statistic or effect. For example, if somebody purchase threeit could be said that they are "stacking" . Alternatively, if a player purchased a , , and a , it could be said that they are "stacking" health.
Stacking items or stacking a particular stat can help to build up a strong point, but it is generally not the best thing to do. For example, while stacking attack damage may seem like the best option for someone who does lots of auto-attacking, that character's damage output would actually benefit more from balancing a variety of both attack damage, attack speed, and armor penetration. In the case of stacking Doran's Rings, each of which gives fairly good stats for an AP Caster, you might benefit more by building towards powerful endgame items which may add more beneficial statistics, such as higher Ability Power or Cooldown Reduction.