Travel backwards or forwards through time, achieved in Star Trek by many means from the slingshot effect to Borg temporal vortices, and policed in later centuries by the Temporal Prime Directive.
Time travel is one of Star Trek's most frequently used ideas. It can happen by accident or by design, and the danger is always the same: a change in the past may erase the future the travellers came from.
The Original Series establishes several methods. The Enterprise is thrown back to the 1960s in Tomorrow Is Yesterday and afterwards uses the slingshot effect around a star on purpose, as it does again in Star Trek IV: The Voyage Home, while The City on the Edge of Forever introduces the Guardian of Forever.
Later methods include the Borg's temporal vortex in Star Trek: First Contact, the Bajoran Orb of Time, chroniton particles, Q's whims and the purpose-built timeships of the 29th century seen in Voyager. Starfleet officers are bound by the Temporal Prime Directive not to interfere with history, and Enterprise centres on a Temporal Cold War.