Time travel

Time travel is, as the name suggests, the process of travelling through time. Devices that can travel through time are collectively known as time machines.

Time machines
There are various types of time machine, the most common and reliable being a TARDIS, although others include SIDRATs and Dalek time machines (DW: The War Games, The Chase). It is also possible to manufacture a time window using warp drive. (DW: The Girl in the Fireplace)

Another method of travelling through time requires travel through a time rift. (TW: Captain Jack Harkness)

Power
Time machines require a large amount of power, which can only be harnessed from a temporal anomaly such as a black hole or a time rift (before the Time War, TARDISes woulf have used the black hole known as the Eye of Harmony, after the war they would use the Cardiff rift). (DW: The Three Doctors, Boom Town)