Multiple realities
(covers information from several alternate timelines)

The temporal variance was an attribute of phenomena that exhibit temporal flux.

Krenim chroniton torpedoes operated on a distinctive temporal variance.

In an alternate timeline, Seven of Nine determined, from a torpedo lodged in USS Voyager's hull, that their temporal variance was 1.47 microseconds. This allowed the Voyager crew to develop temporal shielding to protect themselves against the Krenim attacks. (VOY: "Year of Hell")

In another timeline, Kes was the one who determined the torpedo's temporal variance. (VOY: "Before and After")

An illusory wormhole to Earth, created for the Voyager crew by the telepathic pitcher plant in 2375, showed no signs of temporal variance and was thus deemed safe for travel. (VOY: "Bliss")

This may refer to wormholes that connect between two time periods, such as the Harry Kim wormhole.