Portal

A portal is more commonly known by the slang term "hole" in most regions of the world. As The World is a plane of existence lurking between universes of the Multiverse it is often bombarded by holes that breach a universe's wall and allows access to and fro. In all portals are incredibly rare across the Multiverse, however they are very common in The World. Then usually start small but eventually, little by little, they become larger and larger. Just like plate tectonics slowly moving on a planet like Earth. When a portal has been present long enough the world beyond will begin to influence The World and change its landscape. This could be as simple as the spreading of flora and fauna, or the weather patterns in the area, or even creating huge bodies of water. The edges of a portal appear jagged and when they grow they will grow shard-by-shard. Some portals are miles wide, and miles tall, but none have a "perfect" shape; they are always irregular. From these portals arrive sentient beings from across the Multiverse who set up colonies in The World and mingle with each other. Sometimes these grand, old portals will inexpicably collapse, sealing off The World from the world beyond forever. Some have been sealed off by nervous sentients.

Examples
TBA

External Sources
"A portal in science fiction and fantasy is a magical or technological doorway that connects two distant locations separated by spacetime. It usually consists of two or more gateways, with an object entering one gateway leaving via the other instantaneously." - wikipedia entry on portals.

"Gameplay revolves around the Aperture Science Handheld Portal Device (also known as the "Portal Gun", or ASHPD, the acronym), a handheld device that can create an inter-spatial portal between flat planes, allowing instant travel and a visual and physical connection between any two different locations in 3D space." - Sector W entry on the use of portals in the game named Portal.