Time stream

A time stream was a type of temporal anomaly which, if collided with in a spacecraft under a special set of circumstances, can cause alterations to history.

In 2372, Harry Kim intersected with a time stream while piloting the shuttlecraft Drake in the Delta Quadrant. This had the effect of changing history in that Harry was never assigned to the USS Voyager and instead had a position at Starfleet Headquarters in San Francisco. While in this alternate timeline, Harry met an alien from a temporal inversion fold in the spacetime matrix, who had taken the form of a Human named Cosimo. Cosimo detailed to Harry the events which led to the creation of the alternate timeline, and informed him that the time stream, which wove throughout the galaxy like a thread, could be intersected again, possibly repairing the damage to the timeline. With the help of the alternate timeline-counterpart of Tom Paris, Harry was able to locate and fly into the time stream, recreating the exact conditions which resulted in the change to history in the first place, and undoing the alterations to the timeline. 

Aboard the starship Enterprise, Daniels used the phrase "time stream" to refer to the fluctuations in the timeline that resulted in the alternate timeline that was created when the Temporal Cold War turned into an all-out conflict. 

Timeline reset
Several images of historical importance were seen when the timeline reset. 

Zeitstrom