(covers information from several alternate timelines)
A temporal inversion fold is a type of extradimensional realm.
In an alternate timeline in 2372, Ensign Harry Kim encountered an alien from a temporal inversion fold in the spacetime matrix, who had taken the form of a Human named Cosimo. Cosimo told Harry that the creation of the alternate timeline was a result of Harry intersecting with one of his race's timestreams in a shuttlecraft. Cosimo then detailed to Harry how he could recreate the accident, thereby repairing the damage to the timeline. (VOY: "Non Sequitur")