Theory of quantum physics in which an infinite number of ever-growing alternate realities are created by our choices. In any alternate reality any choice that is made creates universes where each choice is explored. These universes are continually being created to acknowledge any possible decision, like forks in a road.
Parallel universes may be accessed in a limited number of ways and only with great difficulty -- or advanced technology. An alien race once developed the quantum mirror, a device that bridges the metaverse gap and allows access to several different realities. A Stargate's wormhole can also be used to pass from one universe to another if it passes through a black hole where a shaped explosive has detonated at the exact point of the singularity, rupturing the barrier between realities. One universe even had an alternate reality drive installed on its Daedalus, though it is not clear that the ship was ever piloted from one reality to the next in any deliberate or controlled way.
Universes (or "realities") in the metaverse are proximate to one another, with those that are more similar sitting relatively more closely (and, in theory, easier to access). The quantum mirror can be dialed in small increments to identify realities very similar to one another; or in larger increments to find universes more drastically different.
In normal circumstances parallel universes that contain more than one individual of the same identity will cause the displaced person to undergo entropic cascade failure -- a painful quantum disruption at the atomic level. However, if the parallel universes visited are close-enough together (such as universes naturally routed through a black hole) this will not occur.
There But For the Grace of God - Dr. Jackson unknowingly crosses into a parallel universe where the Goa'uld are attacking Earth, and realizes he is witnessing a possible future for his reality.
Point of View - An alternate reality Samantha Carter and Charles Kawalsky escape capture by the Goa'uld by crossing through the quantum mirror. General Hammond later orders the device destroyed.
Ripple Effect - The metaverse is bridged through the Stargate when an alternate reality SG-1 intends to capture a healthy Zero Point Module.
McKay and Mrs. Miller - Rodney and his sister Jeannie create a technology capable of bridging between universes, allowing an alternate version of Dr. McKay -- "Rod" -- to come through to our universe.
The Daedalus Variations - The Atlantis team boards a copy of the Daedalus from a parallel reality, and find that it has a special drive allowing it to skip from one universe to another.