Silences the target, preventing them from casting spells for 4 sec. Against non-players, also interrupts spellcasting and prevents any spell in that school from being cast for 4 sec.
Silence is a Shadow priest talent. It silences the target, preventing them from casting spells for 5 seconds, and acts against an interrupt against non-player targets.
Any spell currently being cast will be prevented from completing when the target is silenced. The spellcasting of non-player targets is also fully interrupted, allowing targets that are immune to silences, such as bosses, to still be interrupted by this ability.
In PvP against casters, this can simply disable them for 5 secs.
In PvP when used against paladins, they can use [Divine Shield] to remove the Silence effect. So preferably it should be used when they have already popped Divine Shield. Warlocks with their Felhunter out can cast its [Devour Magic] on themselves to remove the effect.
In PvE, there are many caster-type mobs that can't be pulled like normal mobs. Pulling with Silence will cause them to come to you to engage in melee, since their ranged attack (magic) is disabled. It can also stop a mob's healing, which is sometimes very useful.
Patch 10.1.0 (2023-05-02): Duration decreased to 3 seconds in PvP combat (was 4 seconds).
Patch 10.0.0 (2022-10-25): Now a row 2 Shadow talent (was a level 29 Shadow ability). Rank 2 effects made baseline.
Patch 9.0.2 (2020-11-17): Rank 2 added at level 56: Increases the range of Silence by 10 yards.
Patch 9.0.1 (2020-10-13): Now learned at level 29 (was 50).
Patch 8.0.1 (2018-07-17): Silence duration reduced to 4 seconds (down from 5). Interrupt duration increased to 4 seconds (up from 3).
Patch 7.0.3 (2016-07-19): No longer available to Discipline. Mana cost removed. Now learned at level 50 (was 52).
Hotfix (2014-12-11):Silence (Discipline) now prevents the target from casting spells for 2 seconds (down from 5 seconds). There is no change to the ability for Shadow Priests.
Patch 6.0.2 (2014-10-14): Now available to Discipline and Shadow priests.
Patch 5.0.4 (2012-08-28): Now a baseline ability for Shadow priests, available at level 52 (was a tier 4 shadow talent).
Patch 4.0.1 (2010-10-12): Silence is now a Tier 4 talent, down from Tier 5. Now has a new icon (was ).
Patch 3.3.0 (2009-12-08): Updated tooltip to include the functionality that this ability also interrupts spellcasting on NPCs.
Patch 3.2.0 (2009-08-04): This ability will also apply a 3 second interrupt effect against non-player controlled targets, making it more versatile against creatures immune to silencing effects.
Patch 3.1.0 (2009-04-14): Range increased to 30 yards (up from 20).
Patch 3.0.8 (2009-01-20): Silence now has diminishing returns.
Patch 1.6.0 (2005-07-12): Several creature spells that silenced players are now properly flagged as silence effects, which will allow talents such as Unbreakable Will to provide a higher chance to resist the effects.
[Last Word]
[Divine Shield] to remove the Silence effect. So preferably it should be used when they have already popped Divine Shield. Warlocks with their Felhunter out can cast its 
[Devour Magic] on themselves to remove the effect.
Patch 10.1.0 (2023-05-02): Duration decreased to 3 seconds in PvP combat (was 4 seconds). 
