FYI... Each NPC have a Weapon Skill formula in his database.
For every point of Defense skill you have above your enemies' Weapon skill, you reduce your chance to be Critically Hit by 0.04% So, Once your Defense skill matches your enemies' Weapon skill, then 25 Defense skill equals 1% of Critical Hit reduction.
As you can see from this, if all you are fighting is a level 80 mob, then your Defense skill already matches his Weapon skill all you need to be immune from his Critical Hits is an extra 125 Defense skill to reduce the 5% Critical Hit chance to zero 
Exp: Our chosen enemy to tank is a level 83 raid boss...
NPCs gain 5 Weapon skill per level, and have a 5% base chance to land a Critical Hit.
To counteract this, we need to reach 415 Defense skill to match the Weapon skill of a level 83 raid boss, and then we need enough extra Defense skill to reduce our enemies' chance to land a Critical Hit from 5% to zero.
So we start out with 5-6% can't remember 
reduction (from Tanking talent) in Critical Hits from our enemies. To make up the last 2%, we need 50 more Defense skill against level 80 enemies, and 65 more Defense skill against level 83 enemies. 65 Defense skill... plus the 400 Defense skill we start out with at level 80... brings us to 465 Defense skill for immunity to Critical Hits from level 83 enemies.
For every 1 Defense Rating on your equipment, gems or enchants, you get 0.423 Defense skill...
Critical Hit immunity for a level 80 player in a Heroic Dungeon is 535 Defense, because mobs in a level 80 heroic 5-man are never higher than level 82.
Critical Hit immunity for a level 80 player against a Raid Boss occurs at 540 Defense. This requires a defense skill of 140, achieved from a combined 689+ Defense Rating from gear, enchants, gems, etc.
Critical Hit immunity for a level 80 player against a Raid Boss can also be achieved by raising Resilience to 5.6%, being a resilience rating of 460.
If the combined critical hit reduction from Defense and Resilience together total 5.6% that will also make the player immune to critical hits from Raid Bosses.
See below for a more detailed discussion of Defense...
Critical Hits are strikes by mobs against the player that deal twice the normal damage. The player's Defense and Resilience are both attributes that reduce the chance of an enemy critically striking. The chance to be critically hit is based on the difference between the attacker's weapon skill and the player's defense skill and resilience %.
A mob has a weapon skill that is calculated by 5 x Level, and a player has a base defense that is 5 x Level. A player does not have any base resilience. Therefore a level 80 mob has 5x80=400 Weapon Skill. A level 80 player has a base defense of 5x80=400 Defense and no resilience.
When an equal level mob is attacking a player, the mob has a 5% chance to critically strike the player. For every level beyond the player, there is an additional 0.2% chance for the mob to critically strike the player. Therefore a level 83 mob (a raid boss) attacking a level 80 player has a 5% + (3 x 0.2%) = 5.6% chance to critically strike the player. A player in the role of a tank for a raid boss must lower his or her chance of being critically struck by 5.6% to avoid the possibility of taking 2x the damage from a single hit. This is where gear bonuses that augment resilience and defense are important.
Each point of defense beyond the player's base reduces the chance to be critically struck by 0.04%. Therefore, to become immune to critical hits dealt from a mob 3 levels higher, a player must have 5.6% / (0.04% per defense) = 140 additional points in their defense skill to avoid a critical strike. Therefore a level 80 player with a base defense of 400 must have 400 + 140 = 540 Defense to become immune to critical hits through defense alone.
Because a single critical hit from a raid boss can easily remove most, if not all, of its target's HP, becoming crit immune is far more important for tanks than anything else until they are immune; even HP granted from high amounts of Stamina is not enough to mitigate a critical hit.
However, resilience may also be used to lower the chance of being critically struck. There is no base resilience for players. Therefore a level 80 player with no additional defense points, will simply need 5.6% resilience to become immune to critical hits dealt by a raid boss.
For a level 70 player, gear that provides +39.4 resilience rating will yield a 1% resilience. And gear that provides 2.36 defense rating will increase the defense skill by 1. For a level 80 player, gear that provides +82.0 resilience rating will yield a 1% resilience. And gear that provides 4.92 defense rating will increase the defense skill by 1. See below for the formula used to derive defense skill from defense rating. See Resilience for a discussion on the formula to calculate % resilience obtained from resilience rating.
Feral druids have Survival of the Fittest, which reduces the inbound crit rate by 6%, making them naturally immune to melee critical hits in PvE.
Reaching the crit cap
This script will show you how much defense rating you need at level 80 to be uncritable in Heroics and Raids. It collects your character's defense and resilience, then returns how many solid points of defense rating are needed to become uncritable. A need of 4.1 defense rating means you must equip 5 points of defense rating to be completely capped, so this script rounds up to the full amount required.
I hope that have benefited
You Bro Faisal
