Proc is a common term used primarily in game programming to refer to an event - a "procedure" - triggered under particular circumstances. For example, in WoW, a particular weapon (that hits many times) might have a 10% chance on each hit to apply a special effect, such as poison damage. When WoW users talk about "how often this weapon procs", they are talking about the likelihood of the special effect occurring.
Proc was originally short for "spec_proc" (spec_proc is short for "special procedure") which is a term used by the original programmer of Circle-MUD, Jeremy Elson. It might have been used as well by the original programmers of diku-MUD as well. Special procedures in Circle-MUD are functions that can be assigned to objects, players, and locations in the world such that each time an event occurs, the special procedure function will be invoked. Special procedures were used in Circle-MUD for a wide variety of purposes: Creating room events when a person typed a specific string of text, causing a weapon or piece of armor to perform a magical action, and even causing a MOB (mobile) to do something that it wouldn't normally do.
Special procedures were a way of creating unique experiences that could not be achieved by simply building the dungeon and populating it with monsters. Special procedures breathed life into these worlds by introducing extra coding that enhanced the gamer's experience without changing the underlying structure or function of the code-base. If you ever played a MUD and were walking around in a dungeon and randomly saw the text "You feel as though you are being watched," that was probably a special procedure.
When developers and players were talking about these special procedures they abbreviated the term to "proc". Over time the noun also became a verb, "proced" ("proc'd" or "procced"), which meant that the special procedure was invoked and performed its action. Most often, players were concerned about their weapons and whether or not the weapon would perform its special attack (a proc), and so proc must have started to take on a narrower meaning for MMORPG players who were somewhat more removed from the core combat engine and flat world of MUDs.
In World of Warcraft
This section concerns content exclusive to World of Warcraft.
In World of Warcraft, players refer to items that have Chance on Use or Chance on Hit effects as having a proc. Most of the time these items are weapons but they can also be armor, rings, trinkets, and even spell effects. How these items trigger their proc(s) depends upon what kind of an item it is.
Guaranteed effects from critical hits (like "after a critical strike your attack speed is increased by 30% for 15 sec") are called proc too (the effect is automatic, but the critical hit is rare.
- Chance on hit: (blasts a target for 40 to 56 Fire damage)
- The active effect has an x% chance to take effect when you successfully attack something with your melee weapon, such as the chance to hurl fireballs at the enemy or to restore mana or health to you. Druids who were shapeshifted could not receive procs from their melee weapon or its enchant until Patch 3.0.2.
- Chance on critical hit: (increase your attack power by 300 for 10 secs)
- Chance on spell hit: (increase your spirit by 222 for 10 secs)
- Chance on spell critical hit: (increase your spell power by 225 for 10 secs)
- Guaranteed on critical melee hit: (Increases your attack speed by 30% for your next 3 swings after dealing a critical strike)
- Similar again, but this time the effect requires a critical hit in order to activate.
- Similar effects also exist for spell critical hits.
- Equip: (strike your melee target with lightning for 250 Nature damage)
- Included here for contrast, "Equip:" effects are not considered "procs," but some of them can provide similar benefits. Further adding to the confusion, a static "Equip:" effect may grant a "Chance on Hit" ability to the wearer, and that granted ability would be considered a proc. For an "Equip:" effect the passive effect, be it increased chance to resist stuns or a chance to return damage to the attacker, will be in effect at all times while this item is equipped and not broken.
- Example: ,
For weapons that have a proc, they usually have a Chance on Hit to either aid the wielder, harm their target(s) or both. Procs can also be added to weapons by enchanting them with certain enchants. The proc-rate of a weapon is the frequency with which the weapon triggers its proc. This varies depending on the nature of the weapon and how it is wielded. Certain epic weapons have a very high chance of proc'ing (30+%), but most players report that wielding one-handed weapons in your off-hand reduces the proc-rate by 50%.
- Main article: Procs per minute
It has been proven however that enchantment procs are based on something called Procs per Minute (PpM, PPM, or ppm). This in essence makes slower weapons, especially on a class like a rogue with instant attacks boosting their attacks per minute like crazy, actually proc more than faster weapons.
The PPM system works as follows: The actual chance each hit with a weapon procs the effect is PPM * weapon speed / 60. Dual wielders should note that all instants (including the extra attack granted by Sword Specialization talent from Warriors & Rogues) always happen with the main hand weapon, severely increasing the number of hits per minute with that weapon. (This also explains as to why people find OH weapons proc much less; they just don't hit as much)
Armor, Rings, and Trinkets
Worn items such as these can either trigger their proc when the wearer is hit in combat, when the wearer casts a spell — offensively or defensively, when the wearer uses the item, or when the wearer hits their opponent with a melee attack. That would tend to make the first proc more valuable to players who get hit quite often, in other words PVPers and warriors, the second proc useful to anyone, and the third proc most useful to melee combatants like rogues.