The word "portal" in science fiction and fantasy generally refers to a technological or magical 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.
Places linked by a portal include a different spot in the same universe (in which case it might be an alternative for teleportation); a parallel world (inter-dimensional portal); the past or the future (time portal); and other planes of existence, such as heaven, hell or other afterworlds. A parallel world, such as C. S. Lewis's Wood between the Worlds in his Chronicles of Narnia, may exist solely to contain multiple portals, perhaps to every parallel world in existence.
Portals are similar to the cosmological concept of a wormhole, and some portals work using wormholes.