Time is a nonspatial continuum in which events occur in a seemingly irreversible transition of the current state to the next as governed by the laws of the local universe. These sequential events form a time stream also known as a timeline.
Deviations in the dominant timeline of any given universe. The Temporal Prime Directive insures that all Starfleet personnel are strictly forbidden from directly interfering with historical events and are required to maintain the current timeline; preventing history from being altered an any way possible. It also restricts people from telling too much about the future, so as not to cause paradoxes or alter the timeline.
Despite this there have been several documented cases of temporal manipulation, either accidental or deliberate, that could create an altered version of a timeline.
- A temporal incursion into the past could cause an alternate chain of events to unfold. However, in the cases where the time travel event was part of a predestination paradox, an alternate timeline was not created. In at least one case, when accidental time travel from 2371 led to the premature death of Gabriel Bell in 2024, an altered future existed for the same amount of time it took for Benjamin Sisko to replace and complete Bell's vital role in the "Bell Riots" of 2024.
- Time travel to the future of a timeline could create an alternate future, where the time traveller had never returned back to the past from the future. Such was the case, for example, when the USS Enterprise-C travelled from 2344 to 2366 through a temporal rift. This however did not always occur, such as in the case when Samuel Clemens visited the 24th century through a temporal distortion created by the Devidians.
- Sometimes objects and people who had travelled through time from the future into the past would vanish when the timeline is altered too far, such as an alternate Captain Jean-Luc Picard who came to the past through an energy vortex in 2365. When objects were pushed completely out of the space-time continuum, they were also erased from history, creating an altered timeline, where the object never came into being. Such was the case with the effect of the main energy beam weapon of the Krenim weapon ship and destroying the interspatial parasites that had infected Captain Jonathan Archer in the 22nd century. More commonly though, changes to the past of a timeline did not cause the objects and people from an alternate future to vanish. Multiple versions would then coexist.
Different versions of a timeline also sometimes coexist as parallel universes. Most active temporal anomalies allow interaction between two different alternate timelines for a moment, making them parallel realities in relation to one another.
- In 2370, Q allowed Picard to shift his consciousness between three parallel timelines, one in the past, one in the present and one in the future from Picard's point of view, in order for him to create an anti-time eruption. The eruption was linking these universes together. When it was sealed into subspace, the coexisting timelines also collapsed.
- In 2371, radiation poisoning and the temporal energy emissions of an artificial quantum singularity of a Romulan Warbird allowed Miles O'Brien to jump between the prime and an alternative timeline in which Deep Space 9 was destroyed. At least during some of the jumps the two timelines coexisted.