Buff

A buff is any status effect given to a player or a minion that gives them a beneficial boost in their performance. Likewise, a buff that gives negative effects is called a debuff. Buffs can boost almost any aspect of a character, such as HP, mana, damage, ability power, and more.

Buffs can be given from any number of things, ranging from the Champion's skills and items (e.g. aura) to slaying neutral monsters found around the map.

Some buffs can have a (usually capped) number of stacks associated with it, where the more stacks it has, the more the positive effect is enhanced. Other buffs are unstackable, meaning that repeated applications of the buff to the same target will not grant him any increased effect but will only cause the buff duration to reset.

Buffs are in fact usually only temporary, but some buffs may last indefinitely (such as 's boost to her attack range) or until player death (such as Oracle's Elixir and stacking buffs such as or 's Feast stacks).

Some jungle monsters, when slained, grant a buff to its killer (for example, the gives the  buff). In this case, the monster usually has some swirling colored runes around it. Other monsters, like, grant a buff to the whole killing team instead.

Other Meanings
Sometime, positive changes to Champions or Items in the Patch Notes can also be called buffs, while negative changes can be called nerfs instead.