From Diablo Wiki
Proc is a term that refers a skill, item, or other effect that is activated automatically, usually on some percentage chance. An effect such as "2% chance of knockback" could be said to have a 2% chance of proc'ing.
Procs are mostly commonly seen in Diablo III when active or passive skills trigger an effect with every critical hit, such as the Wizard's Critical Mass passive skill.
Procs are widely used in World of Warcraft skills, which no doubt influenced the Diablo III developers in their design.
- See more Reference terms.
 Procs in Diablo III
In the case of Energy Twister, specifically Wicked Wind, the 0.25 proc coefficient causes the skill to generate more procs in a given time period than any other skill. Currently, this is used in combination with Critical Mass to lower the cooldown on skills like Frost Nova and Diamond Skin. By reducing the proc coefficient from 0.25 to 0.125, the build still works and remains fairly strong, but it won’t be quite as good as it is now. (For those with extremely high Crit rates, you may not even notice much difference, but I wanted to call it out anyway. )
The term "proc" is a programming term that refers to an event triggered under particular circumstances, or being "processed".
Proc is also sometimes short for "spec_proc" ("special procedure"), used by the original programmer of Circle-MUD, Jeremy Elson and possibly the original programmers of diku-MUD as well. These "special procedures" are functions assigned to objects, players, and locations to invoke a "special procedure" when a specific event occurs. These were used in a variety of ways in Circle-MUD, and might have originated the term.
When developers and players were talking about "special procedures" it was abbreviated to "proc". It evolved to a verb ("proced", "proc'd" or "procced"), indicating the procedure was activated.
 Origin Explanation
- Short for spec_proc (special procedure), which is a bit of code triggered to cover a special case that the default code doesn’t handle.
- In the older muds there was almost no variation between what a given object could do. For example all weapons used the weapon type, then you could specify damage type (was it a sword or a mace), damage ranges, and so on.
- To get the weapon to do anything special, you had limited choices. Depending on the architecture, you could attach a spell to be cast, or could attach a script if the code supported it. In the codebases that Brad & co. played, the devs could not script, so the codebase allowed a pointer to a special hardcoded procedure to be entered in the weapon data.
- “Proc” is almost entirely Everquest slang... Even in the muds, it wasn’t that widely used because only some codebases used the term. It took EQ publicizing the inherited term to make it common knowledge.