Time travel was a form of passage through the spacetime continuum, by moving to different points or destinations, "backward" or "forward", via a "fourth dimension" (time), often while moving through space. Temporal displacement also refers to time travel.
Because of the risks inherent to altering the outcome of historical events, via figures from one era interacting with another (usually the past), time travel was closely monitored, regulated and discouraged. In the Federation, the Department of Temporal Investigations was charged with this task.
Despite the risks, time travel was a fairly common occurrence. Individuals and starships, in the Federation and elsewhere, made many trips through time. Crew-members on Starfleet vessels, most notably the starships Enterprise, interacted repeatedly with figures and events in other temporal eras, resulting in the creation of alternate timelines.