Active Mitigation

Active mitigation is a model for tanking, first implemented with Mists of Pandaria, which requires tanks to play an active role in their own damage mitigation.

Design (Mists of Pandaria)

The active mitigation model was first discussed during the lead-up to Mists of Pandaria, involving tanks in their own survival at a much higher granularity than the previous system. The removal of threat as a nail-biting concern (after the first few seconds of the fight) meant that tanks' most common life/death decisions were about when to spend (relatively-) long-cooldown defensive abilities.

In contrast, active mitigation gives tanks abilities that are used frequently, but affect their long-term survival in borderline fights. A skilled tank is the one who continuously makes choices that reduce the amount of damage they take, or reduce the amount of mana healers spend upon them and the raid. Unskilled tanks given the same abilities will use them at the "wrong" time, or at suboptimal moments that place a larger load on the healers and the rest of the raid, and can lead to a wipe on progression content.

Death knights were the inspiration/laboratory for active mitigation through their [Death Strike] ability which included a healing component whose exact mechanics rewarded tanks who could time their usage against incoming damage. This decision-making happens regularly as the tank is in combat and tanks can distinguish themselves by using this ability more skilfully than others.

With Mists of Pandaria the various tanks gained short-cooldown abilities that embody active mitigation. Tanks who use these abilities optimally will be able to overcome challenges that would defeat a less skilled tank.

Design (Legion)

Legion continues the active mitigation model from Mists of Pandaria, but the term "Active Mitigation" is now differentiated from the broader active mitigation model.^[1]

The term "Active Mitigation" now refers to a single, specific ability that a Tank must use as a counter to specific Boss abilities. These Boss abilities are designated as Mitigation Checks. If a tank fails to use Active Mitigation against a Boss’s Mitigation Check, they will receive an additional negative side effect. Some examples of Mitigation Checks in Legion Dungeons are:

The second Boss in Black Rook Hold has "Vengeful Shear"; if she lands it on you and you don’t actively mitigate it, you get a stacking debuff that makes you take extra damage.
The last Boss in Neltharion’s Lair has "Molten Crash"; if you fail to actively mitigate it, you fly far, far away when he hits you.
God King Skovald in the Halls of Valor has "Savage Blade"; If you fail to actively mitigate it, you take a bleed over time.
The first Boss in Vault of the Wardens has "Dark Strikes"; if you fail to actively mitigate it, the boss gets a damage absorb shield.

Every Tank in Legion has only one ability that counts as "Active Mitigation" against Mitigation Checks.

Vengeance Demon Hunter — Demon Spikes
Guardian Druid — Ironfur
Brewmaster Monk — Ironskin Brew
Protection Paladin — Shield of the Righteous
Protection Warrior — Shield Block
Blood Death Knight — Blood Shield / Death Strike / Marrowrend ("recently used Death Strike" buff gained after using Death Strike or Marrowrend)

There are other "active abilities" that "mitigate" but they are not considered "Active Mitigation" as they do not counter Mitigation Checks. For instance:

A Warrior's Active Mitigation is "Shield Block," while "Ignore Pain" is not Active Mitigation and does NOT count for Mitigation Checks, even against attacks that deal purely magic damage.
A Demon Hunter's Active Mitigation is "Demon Spikes," while "Empower Wards" is not Active Mitigation and does NOT count for Mitigation Checks, even against attacks that deal purely magic damage.