There are numerous stats that can improve your survivability in Path of Exile. The most important two are Life and Resistances, with Armour, Evasion, and Energy Shield offering secondary defenses.
Note that as an in-game keyword, Defenses refers specifically to Armour, Evasion, and Energy Shield (and not to Life, Resistances, or other defensive stats).
Note that "armour" can refer both to items in your Glove, Chest, Helmet, and Boot slots, and to the stat Armour, a rare example of real ambiguity in Path of Exile's wording. It should be clear from context throughout most of this guide which one is meant, but I've made an effort to capitalize Armour when I mean the stat.
Next to raw Life, the most important thing you're looking for on your equipment is Resistance. It comes in four forms: Fire, Cold, Lightning, and (more rarely) Chaos Resistance. Remember from earlier sections that the word "Elemental" means "Fire, Cold, and Lightning", so items that give "+X% to all Elemental Resistances" will not give Chaos Resistance.
Resistances reduce the amount of damage you take from that type of damage by a flat percentage. If you have 30% Fire Resistance, for example, you will take 30% less Fire Damage.
Resistances cap at 75%. To start, your base Resistances are 0%. Later in the game you'll suffer a penalty and they'll start at -30%, then at -60%, and you'll need better equipment to compensate. There is no way to avoid this penalty, which you incur as part of the main storyline. Some items can increase this cap, up to a hard maximum of 90% maximum resist.
Capping resistances early and keeping them there is vitally important to any build - when a new player asks why they're dying, "how are your resistances" is the first question any experienced player will ask. ("How much Life do you have?" is the second.)
The Three Types of Defense
In Path of Exile, Defenses refer to the three stats present on armour equipment: Armour, Evasion, and Energy Shield. Any equipment you find for your Head, Chest, Hand, or Foot slots will generally have one or two of these defenses: Strength equipment has Armour, Dexterity equipment has Evasion, and Intelligence equipment has Energy Shield. Items that require a mix of Attributes will have two different Defenses present on them, but offer less of each than pure equipment.
It is a common mistake for new players to over-emphasize these. They're not irrelevant, but again, Life and Resistances should come first for a new player. If necessary, do not be afraid to use a piece of equipment not typical for your Attributes if it has good Life and Resistances or desirable socket colors.
Armour reduces the Physical damage from Hits. Armour does nothing against non-Physical damage or against things that do not Hit (like Damage over Time effects). Note that Spells can deal Physical damage in Path of Exile, and Attacks can deal non-Physical damage: Armour would reduce a Physical Spell's damage, but not a non-Physical Attack's damage.
The math behind Armour is complex, but is not necessary for most players to handle in detail. Roughly speaking, Armour's effect on damage scales between a roughly percentage based amount of small Hits to a flat reduction on very large ones. Armour can reduce the value of a Hit by at most one-tenth your armor value (so 10 Armour will reduce each Hit by at most 1 damage), and reduces larger hits by a smaller percentage than smaller ones.
Mechnically speaking, Armour prevents a percentage of Physical damage from the Hit, based on how large the Hit is relative to your Armour. Other sources of "% additional Physical Damage Reduction when Hit" are then added to the amount computed from Armour, meaning that such effects are effectively much stronger the more Armour you have (and Armour is stronger the more such effects you have). This total Physical Damage Reduction caps at 95% of any given hit prevented, including both Armour and other sources.
Armour is associated with Strength, so nodes affecting it are found in the Strength area of the Passive tree at the lower-left and Armour equipment tends to have red sockets.
Note that, confusingly, 'Armour' is also Path of Exile's term for non-weapon equipment. Which one we mean is usually clear from context.
Evasion gives you a chance to avoid Attacks. If successful, you will avoid both the damage and other effects of the Attack. Evasion does nothing against Spells, and does not reduce the damage of Attacks that do successfully Hit.
Your chance to Evade an attack depends both on your Evasion and on your attacker's Accuracy. The exact formula is complicated, but a rough estimate based on the average Accuracy of monsters of your level can be found on your character panel.
Evasion is not completely random, and has bad-luck protection built in. What this means is that if you have, say, a 60% chance to Evade enemies, you can expect to, on average, take 60% less damage from their attacks over a long period of time. However, it is possible to fail to Evade several consecutive hits from a boss if other enemies attack you between their hits, resulting in a potentially fatal spike of damage, so a solid Life pool is still important.
Evasion also effectively reduces your chance to be Critically Hit by Attacks, because Path of Exile's mechanics require Attacks to effectively 'hit twice' in order to land a Critical Hit.
Note that Path of Exile also has a mechanic called "Dodge": Dodge is completely separate from Evasion and does not directly interact with it in any way, although it has a similar effect (simply allowing you to avoid damage). Unlike Evasion, Dodge is a pure % chance that has nothing to do with your attacker's stats, and is truly random without bad-luck protection.
Evasion is associated with Dexterity, so nodes affecting it are mostly found in the Dexterity area of the Passive Tree at the lower-right and Evasion equipment tends to have green sockets.
Energy Shield acts as a secondary life pool that is drained by taking damage. You'll only start taking damage on your Life once your Energy Shield is depleted. Energy Shield can Recharge after not taking damage for a short period of time (by default, 2 seconds).
Energy Shield is displayed as a bright-blue overlay over your Life orb at the bottom-left of your screen. The higher your Energy Shield is relative to your Life, the more of the Life orb your Energy Shield will overlay.
Chaos Damage bypasses Energy Shield entirely, meaning that characters with high Energy Shield but low Life are very vulnerable to it.
Unlike Armour and Evasion, Energy Shield can be used as a replacement for Life if you have enough of it. Builds that do this usually take Chaos Inoculation, a Keystone Passive located near the top-right of the Passive Tree, which reduces your Life to 1 but makes you entirely immune to Chaos Damage (which would kill you immediately if you only had 1 Life, since it bypasses Energy Shield).
Energy Shield is associated with Intelligence, so nodes affecting it are mostly found in the Intelligence area of the Passive Tree at the top and Energy Shield equipment tends to have blue sockets.
Using Defenses Properly
Life is Life
For most builds, Life and Resistance are far more important than Defenses. You cannot, for example, simply build an Armour character with no Life bonuses and expect to live.
Armour and Evasion are secondary defenses, used to reduce your damage intake enough that you can easily heal it up. If you feel you're short on one of these, consider swapping in a Jade Flask (evasion) or Granite Flask (armour), particularly one with an affix that grants X% Increased [Evasion or Armour] During Flask Effect. Few builds emphasize Armour or Evasion as core stats.
Energy Shield, on the other hand, deserves some special mention.
Energy Shield Builds
Because Energy Shield acts effectively as a secondary Life pool, it is possible to build a character around Energy Shield instead of Life. In order to do this, you need large bonuses to Energy Shield on your passive tree - generally something like 250%-300% Increased Energy Shield by the endgame - and equipment with large amounts of Energy Shield on it. If you're new, high-Energy Shield gear is hard to come by, which is why none of the builds recommended in this guide rely on it as a primary defense.
Such builds generally come in one of a few types:
- Chaos Inoculation builds stack Energy Shield up as high as they can get it, then pick up the Chaos Inoculation Keystone Passive on the Passive tree to make themselves immune to Chaos damage (which bypasses Energy Shield) at the cost of setting their Life to only 1. In these builds, Energy Shield really does act just like Life, except that it recharges when you're not taking hits.
- "Low-Life" builds use Energy Shield to protect their Life, then block off ("Reserve") most of their life with Auras and other effects. As a result, they're always "On Low Life" in game-mechanics terms, which can yield some powerful benefits from certain items and from the Pain Attunement Keystone Passive. To do this properly, they generally need some effect that stops Chaos Damage from bypassing their Energy Shield to hit their limited Life total. A few Unique items come with such a bonus.
- Hybrid builds that use very high Chaos Resistance (often from the Occultist ascendancy) to run Energy Shield without needing to use Chaos Inoculation.
If you're new to the game, Energy Shield is recommended only for spellcaster builds near the top of the passive tree, and even then can be somewhat hard to use properly. Life builds are more common.