A passage through time opened by a Borg sphere using chronometric particles in Star Trek: First Contact, carrying the Borg from 2373 to 2063 and drawing the Enterprise-E after them in its wake.
A temporal vortex is an artificial opening through which a ship can travel into the past or future. The one seen in Star Trek: First Contact is created by the Borg, who generate it by emitting chronometric particles.
After the Borg cube is destroyed above Earth in 2373, a sphere escapes from it and opens a vortex. The Enterprise-E, close behind, is caught in the temporal wake and so is shielded when history changes around it: the crew see an Earth wholly assimilated, with nine billion Borg. Picard follows the sphere through to April 2063, where the Borg are attacking Zefram Cochrane's missile complex in Montana to prevent first contact with the Vulcans.
Once the Phoenix has flown and the Borg Queen is dead, the crew reproduce the vortex themselves to return to the 24th century. Enterprise's Regeneration later shows wreckage of the sphere found in the Arctic.