The time vortex is the focal point of all timelines.
The time portal known as the Guardian of Forever allowed access to the time vortex. Other dimensions and points in time and space could be observed and traveled to through the use of the vortex. (TOS: "The City on the Edge of Forever")
The experience of using the Guardian of Forever for time travel was referred to as being in the time vortex by Captain James T. Kirk and historian Erickson. When historians used the time vortex for replaying the history of Vulcan, it caused a predestination paradox to unravel in 2269. In the prime timeline, Spock saved his own life as a child in 2239. Spock, however, was only motivated to do so after his away team returned from the past of Orion to an alternate timeline where the viewing of history had revealed that Spock died as a child and had to be saved. (TAS: "Yesteryear")