Also known as Dimensional Rift, it's a phenomenon where an opening forms and connects two worlds, allowing various matters and entities to cross over from one world to another. It's named as such because of the way the opening appears out of the blue and how the shape resembles an earth fissure. It either manifests naturally due to the fragile structure of the world itself (in this case, multiple World Fissures may form simultaneously) or is deliberately created by those who possess the ability to leap between worlds.
Man-made World Fissures are usually temporary and they're mostly formed as quick getaways from dangerous situations or for entities to slip into another realm. Naturally-formed World Fissures might take a while to fade on their own, but there are cases where World Fissures persist unless the conditions to close them are met.