Proc is a term inherited from MUDs, multi-user dungeons -- a precursor to the MMOGs of today. When an item in a MUD would have a special effect attached to it, the MUD's programming would use "SPEC_PROC", short for "special procedure", to attach the effect to the item by linking it to a procedure programmed elsewhere in the MUD's code. Because of this internal naming, "Spec Procs", and later just "Procs", refer to any unusual effect on an item.
As multiuser gaming on the Internet became more mainstream, the etymology of many terms inherited from the earlier days of online gaming became blurred, and "proc" was no exception. Even today in the face of easily verifiable evidence showing the true origin of the term, some people continue to attribute the origin of the term as a broken acronym meaning "Programmed Random Occurrence", or an abbreviation to the similar term "process".
These days, "proc" refers to both the effect attached to an item, and the action of that effect occurring; as in "The proc on Silent Fang will silence your target for 6 seconds", and "The paladin spent the fight waiting for Seal of Command to proc on his opponent".
In World of Warcraft, "procs per hit" is the percent chance of a proc occurring each hit. This can be an attack you launch or an attack launched on you. This may seem obvious, but it's a good term to have in contrast to the less intuitive procs per minute. Procs per hit benefit greatly from faster hits.
Procs per minute scales procs per hit according to weapon speed. Here's the formula:
- Procs per hit = (Weapon speed) / (60 / Procs per minute)
which is the same as:
- Procs per hit = Weapon speed * Procs per minute / 60
This gives the percentage chance that the proc will go off each hit. An understandable misconception is that procs per minute guarantees that exactly some amount of procs will occur each minute of combat. This is not the case, as procs per minute is merely a statistical average, not a guarantee.
Current List of ppm:
Crusader: 1
Mongoose: 1
Lifestealing: 6
Fiery: 6
Seal of Command: 6.8
Demonslaying: ??
Unholy: ??
Icy: 1.6
Effects from weapons and gear:
Halberd of Smiting: 0.4
Instant attacks
The procs-per-minute rating of a given weapon ability assumes that only auto-attacks ("white damage" attacks) are made with that weapon. In practice, an ability on the weapon equipped in the main hand can proc more often than its PPM rating, due to the fact that instant attacks (Sinister Strike, Overpower, etc.) are all assumed to be made with this weapon.
As an example, say a warrior has a Crusader enchant on the weapon in his main hand (with a choice between a weapon with a 2.0 speed and a weapon with 2.5 speed), and is involved in a long fight where he has essentially unlimited rage and (miraculously) never misses his target.
Weapon 1 has a speed of 2.0 sec and is enchanted with Crusader which has a rated PPM = 1. Hence, the chance for Crusader to proc on any given hit with this weapon is 1/30.
Weapon 2 has a speed of 2.5 sec and is also enchanted with Crusader. The chance for Crusader to proc on any given hit with this weapon will be 1/24.
Every minute, the warrior does 10 Bloodthirsts*, 6 Whirlwinds, and 4 Overpowers, for a total of 20 instant attacks. With weapon 1, he will do 30 white hits, for a total of 50 Hits per minute. With weapon 2, he will do 24 white hits.
For weapon 1 the chance to proc is 1/30, the warrior does a total of 30 normal hits + 20 "special" hits per minute which makes the actual PPM 50/30 = 1.67.
For weapon 2 the chance to proc is 1/24, the warrior does a total of 24 normal hits + 20 "special" hits per minute which makes the actual PPM 44/24 = 1.83.
Therefore, the slower the weapon you wield in the main hand, the more often any PPM-rated ability on it will proc.
All these calculations are made with no flurry included.
* It is not clear whether Bloodthirst actually counts as an attack with the wielded weapon, since it does not require a melee weapon to be wielded when used.
† As of Patch 2.3, Whirlwind is confirmed to do damage with a secondary weapon. No definitive evidence that Bloodthirst or Overpower apply to secondary weapon attacks.