Proc
Procing or proc is a term used with MMO (massively-multiplayer online role-playing game) and other games where an item used by the player randomly activates a special ability. In the picture of the Dragon's Call sword in the game World of Warcraft, you can see that the proc (chance on hit) summons an Emerald Dragon Whelp. Depending on the item, it's also possible that an item could proc multiple times.
A proc usually occurs as a percentage chance from use of a player's abilities in the game, like a specific type of attack. The most common type of proc is one that adds additional stats to a character, helping make them more powerful for a short time. Other procs, like that pictured above, may provide extra abilities to a player or activate visual changes to a player's armor or weapon.