Metaverse

Theory of quantum physics in which an infinite number of ever-growing alternate realities are created. 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. An alien race once developed the quantum mirror, a device which bridges the metaverse gap and allows passage between temporal realities. A Stargate's wormhole, if passing through a black hole where a shaped explosive has detonated at the exact point of the singularity will also rupture the metaverse and allow passage to the outgoing location in a different reality.

In normal circumstances, parallel universes that contain more than one individual of the same identity will undergo entropic cascade failure. However, if the parallel universes visited are close-enough together (such as universes naturally routed through a black hole) this will not occur.

KEY EPISODES

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.