A Starfleet principle that forbids officers from changing history or passing on knowledge of the future, invoked repeatedly aboard Voyager and enforced in later centuries by timeship crews.
The Temporal Prime Directive extends the idea of non-interference to time itself. Any Starfleet officer who travels into the past must avoid changing events, and anyone who learns of the future must keep that knowledge to themselves so that history unfolds as it should.
The rule is mostly heard aboard Voyager. In Relativity the 29th-century timeship crew remind Seven of Nine and Janeway of it before returning them to their own time. In Shattered, Chakotay cites it when a Janeway from seven years earlier presses him for details of what lies ahead, and in Endgame the older Admiral Janeway brushes it aside altogether in order to bring her crew home sooner.
The directive belongs to the same family of concerns as the Department of Temporal Investigations seen in Trials and Tribble-ations, which questions Sisko about his trip to the 23rd century.