Cooldown reduction

Cooldown reduction (abbreviated as CDR) is a champion statistic that reduces the cooldown of abilities, or the amount of time you must wait before an ability can be used again by a percentage, once you activated it.

Cooldown reduction is granted by items, buffs, runes and masteries, as well as some passive abilities.

Cooldown reduction works additively. If you have a 6% cooldown reduction from masteries, and a 10% cooldown reduction from you will have a 16% cooldown reduction.

Cooldown reduction is unique among the same item. So 2 (+15%) will only grant the bonus of +15%, but a  (+15%) and a  (+10%) will grant a +25% bonus.

Cooldown reduction cannot be increased beyond 40% by absolutely any means.

Cooldown reduction is determined at the moment an ability is activated. Increasing/decreasing your cooldown reduction while the skill is on cooldown will not affect the cooldown.

Cooldown reduction does not apply for Item abilities such as the ones on Deathfire Grasp or Hextech Gunblade

Items
There are several items which reduce the cooldown of your abilities. Cooldown Reduction granted by an item is always a unique passive.

Champion abilities

 * increases cooldown reduction by 10/15/20%.
 * increases cooldown reduction by 2/4/6/8/10%.
 * increases cooldown reduction by 3/6/9/12/15%.

Masteries

 * increases cooldown reduction by 0.75/1.5/2.25/3%.
 * increases Cooldown reduction by 2/4/6%.

Runes

 * and runes increase cooldown reduction.

Neutral buffs

 * The buff increases cooldown reduction by (16+level*0.5)%.
 * The buff increases cooldown reduction by 10%.

Lowering cooldowns
Some champions have abilities that decrease their cooldowns by a flat amount or entirely refresh their cooldown. By using those, a champion can disregard the 40% cooldown reduction cap.

Flat reduction

 * cooldown is reduced by 2 seconds each time a nearby enemy unit dies.
 * cooldown is reduced by 0.5 seconds each time Amumu is hit by an attack.
 * cooldown is reduced to 0.5 seconds if the target is poisoned.
 * reduces all of Ezreal's cooldowns by 1 second if it strikes an enemy unit.
 * cooldown is reduced by 1 second for every autoattack.
 * reduces all her cooldowns by 15 seconds whenever she gets a kill or an assist.
 * innate cooldown is reduced by 1 second for every autoattack.
 * cooldown is reduced by 6 seconds if he manages to pick up his axe after using the ability.
 * allows him to reduce all of his other abilities' cooldowns by 1 when he uses an ability.
 * cooldown is reduced by 2 seconds every time an enemy champion hits Shen with a basic attack.
 * cooldown is reduced by 0.5 seconds for every autoattack.
 * allows Skarner to reduces all of his abilities' cooldowns by 1 second for each successful autoattack on an enemy champion and 0.5 seconds for each successful autoattack on a non-champion target.
 * cooldown is reduced by 1 for every autoattack and 3 seconds with every autoattack on a champion.
 * , when active, reduces all of his other abilities' cooldowns by 1 second with every basic attack.
 * on activation, reduces all of Zilean's other cooldowns by 10 seconds.

Cooldown refreshing

 * refreshes all Master Yi's cooldowns whenever he kills a champion while under the effect of the skill. Additionally, his cooldowns are refreshed by half whenever he gets an assist.
 * cooldown is refreshed if it kills a unit.
 * cooldown is refreshed whenever she gets a kill or an assist.
 * cooldown is refreshed every time an enemy champion dies.