The Temporal Prime Directive is a principle in the Star Trek universe, similar to the standard Prime Directive. Its purpose is to prevent a time traveller from interfering in the natural development of a timeline. This can include altering events or informing those in the past prematurely to change an outcome.
Adhereing to the TPD prevents the creation of new timelines, since each incursion creates a new one. Such changes are sometimes rationalized as what was 'supposed' to happen. A blatant disregard for the TPD was shown throughout the Temporal Cold War.
The TPD was formally created at least as early as the 2370s, and was enforced through an agency of Starfleet called Temporal Investigations. By the 29th century, this body became the Temporal Integrity Commission, which monitored and resricted deviations from the natural flow of history.
The benefit of applying the TPD would be consolidating timelines to a bare minimum. This would avoid undesirable circumstances for those in timelines affected with malicious intent, but also presupposes that the original timeline is worth preservation.
