The multiverse theory of quantum physics posits the existence of parallel universes, an infinite number of ever-growing alternate realities, each existing and evolving concurrently with our own. The theory holds that anything that can happen will happen, if not in this reality, then in another. We're part of a meta-verse in which individual sub-universes are continually being generated, some of which are very different, and some of which are almost identical. For every possible universe there are infinite variations diverging at every choice we make like forks in the road.
SG-1 has had several experiences with alternate realities. During the mission to P3R-233, Daniel was drawn through the quantum mirror and into a parallel universe in which Earth was under attack by the Goa'uld, but he was able to return with a warning that prevented the same fate for his own world. After the quantum mirror had been moved to Area 51, an alternate Carter and Kawalsky entered our reality, seeking help to defend their own world against the Goa'uld. Six years later, an SG-1 team from a parallel universe deliberately used a wormhole passing through a black hole to create a subspace rupture allowing them to cross into our reality, however their plan inadvertently resulted in a ripple effect, bringing multiple SG teams through the subspace rift. The following year, Carter from a parallel universe was experimenting with the creation of inter-universal bridges for the collection of energy, but an accident in the lab drew Carter from our universe into the alternate reality. Travel through parallel universes brings with it the danger of temporal entropic cascade failure, in which a duplicate individual existing in the same reality causes a temporal distortion and must be returned to his own universe.
The multiverse theory is not to be confused with M-theory, which states that there are a fixed number of alternate dimensions existing in parallel with our own physical reality, dimensions that can be accessed by shifting in and out of phase. Within the stargate universe, alternate reality, alternate dimension, and time travel may be considered the SG-1 trifecta.
Cross Reference: Alternate Reality - P3R-233, Alternate Reality - Quantum Mirror, Alternate Reality - Ripple Effect, Alternate Reality - Universal Bridge, Entropic Cascade Failure, Multiverse Theory
Episode Reference: There But For the Grace of God, Point of View, Ripple Effect, The Road Not Taken