A wormhole, also known as an Einstein-Rosen bridge, is a postulated method, within the general theory of relativity, of moving from one point in space to another without crossing the space between. Wormholes are a popular feature of science fiction as they allow interstellar travel within human timescales.
Wormholes seem to be popular in progressive metal.
There is an episode of Invader Zim wherein Zim, in order to get rid of Dib and his horrible classmates once and for all, utilizes a wormhole to send Dib and the other Skoolkids on a one-way busride to an alternate dimension containing a room with a moose. However, Dib discovers Zim's plan, and taking advantage of a fork in the wormhole, is able to transport the bus back to Earth.
In the movie Event Horizon, the titular ship is designed to create an artificial wormhole. However, the wormhole doesn't lead to anywhere in the known universe, but to an alternate, horrific reality.
In the upcoming movie Railhead, based on a novel by Philip Reeve scheduled for release in 2015, humans live in a future where trains run through space via portals.
In the television series Fringe, the main storyline is the investigation of an unusual series of events and scientific experiments called the Pattern. In the second-season episode "Peter" it's revealed that the root cause of the Pattern was an incident in 1985 where Dr. Walter Bishop opened a wormhole into an alternate universe so that he may cure the alternate version of his terminally ill son, Peter (who had died in our universe). By crossing the wormhole, Dr. Bishop disrupted the fundamental laws of nature and weakened the fabric of space-time, causing incalculable destruction in the alternate universe and forcing them to seek a way to repair the damage caused and save their existence.