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Collision refers to a game physics rule that objects that can not occupy the same location in a game.

There is no player collision in Diablo III, which means that characters, minions, and their spells can not block each other from passing through even the narrowest hallways. Even spells like Zombie Wall will not block the movement of players.

Note that players and their minions or spells vs. monsters do have collision. Players can not run through most types of monsters and some obstructing monster spells (nor monsters through players), and can thus be trapped by large monsters, groups of enemies in tight areas, and other such painful fates.