A transporter modification, later a hand-held unit, developed by the mirror universe's Miles O'Brien that allowed people to cross between that universe and Deep Space 9 at will.
After the crossover of Kirk's time, transporters in the mirror universe were redesigned to stop accidental travel between realities. A century later the rebel engineer known as Smiley O'Brien found a way to do it deliberately.
In the Deep Space Nine episode Through the Looking Glass he uses it to kidnap Benjamin Sisko, who must pose as his dead counterpart. In Shattered Mirror, the mirror Jennifer Sisko uses it to lure Jake across.
By Resurrection the technology is a small portable device, with which the mirror Bareil Antos arrives on the station's Ops. In The Emperor's New Cloak, Grand Nagus Zek takes one to the other side to open trade and is held hostage, forcing Quark and Rom to follow with a stolen Klingon cloaking device.