Quantum incursion
From Memory Alpha, the free Star Trek reference.
A quantum incursion occurs when objects or lifeforms from one quantum reality cross over into a separate quantum reality.
In 2370, Worf was the center of multiple shifts between quantum realities. In order to stop the shifting, the USS Enterprise-D attempted to locate his original quantum reality and return him to it. The need for this was driven not just by the displacement of Worf, but by an exponentially increasing number of quantum incursions in the region of space where the shift in realities had first occurred. The ongoing incursions threatened to disrupt the space-time continuum unless they were stopped by Worf's return to his original reality. (TNG: "Parallels")
