Phase variance
From Memory Alpha, the free Star Trek reference.
A phase variance is a term applicable to phenomena or technologies that show wave-like properties.
When attempting to transport Telek R'Mor through a micro-wormhole in 2371, the USS Voyager crew were hindered by a strange phase variance in the radiation stream, caused by the fact that the two ends of the wormhole were in separate time periods. (VOY: "Eye of the Needle")
The interphase cloaking devices used by the Voth produce a localized spatial fluctuation with modulating phase variance. This allowed the Voyager crew to detect Forra Gegen and Tova Veer aboard their ship in 2373. (VOY: "Distant Origin")
In 2374, B'Elanna Torres determined that adjusting the sensors to a phase variance of 0.15 would allow them to detect the cloaked Srivani that had infiltrated USS Voyager. The Doctor later adjusted Seven of Nine's sensory nodes to do the same. (VOY: "Scientific Method")
Later that year, Torres speculated that a phase variance in the transporter was responsible for igniting a sample of protomatter that Neelix had attempted to collect from a class 1 nebula. (VOY: "Mortal Coil")
When the USS Voyager constructed an experimental quantum slipstream drive in 2375, a 0.42 phase variance in the slipstream threshold emerged during testing, enough to overload the quantum matrix and force the starship out of slipstream with catastrophic results. After much discussion, Harry Kim proposed having the Delta Flyer stay ahead of Voyager, mapping the slipstream threshold as it forms and relaying phase corrections to dampen the variance.
During the initial flight, the phase variance emerged as in the tests. However, the phase corrections were unable to compensate for the rising phase variance, causing the failure of Voyager's helm controls and leaving the slipstream on the verge of collapse. Seven of Nine received a new set of phase corrections through her cranial implants that deactivated the drive. Along with the corrections was a message from a future Harry Kim in an alternate timeline.
In the older Kim's timeline, the phase variance caused the slipstream to collapse, throwing Voyager into normal space to crash on a planet in the Takara sector, with the loss of all aboard. (VOY: "Timeless")
