An interdimensional rift is a phenomenon which can allow for transit to realms outside of the normal spacetime continuum.
In 2373, the Borg used a series of quantum singularities as interdimensional rifts in order to gain access to fluidic space. Their plan backfired, however, when they found that realm's only inhabitants, Species 8472, to be resistant to assimilation. This ignited a war between the two powers, into which the USS Voyager was drawn while attempting to cross Borg space. Seven of Nine was able to modify Voyager's navigational deflector to emit a resonant graviton beam to open a singularity to fluidic space, where the crew was able to halt Species 8472's invasion with a series of biomolecular warheads. (VOY: "Scorpion", "Scorpion, Part II")