Unique

Unique is a term used to describe an item effect. "Stacking" -- that is, using more than one of the same item -- items with unique effects will not result in multiple applications of the item effects.

For example, Zhonya's Ring has three effects :
 * +120 Ability Power
 * UNIQUE PASSIVE: Increases ability power by 25%
 * UNIQUE ACTIVE: Places your champion into Stasis for 2 seconds, rendering you invulnerable and untargetable but unable to take any actions

But since the last two effects are unique, two Zhonya's Rings will result in only the first effect stacking. In other words, the effects of two rings would be:
 * +240 Ability Power
 * UNIQUE PASSIVE: Increases ability power by 25%
 * UNIQUE ACTIVE: Places your champion into Stasis for 2 seconds, rendering you invulnerable and untargetable but unable to take any actions

Notice that despite having two rings, you will only get one Ability Power increase of 25% and only one active stasis effect.