The past was the set of all events that occurred before a given point in time, preceeding one's present, which was itself followed by the future. In regards to time travellers, the concept of past, present and future was relative.
Following Rose Tyler's trip to the year 5,000,000,000, far in her relative future, the Ninth Doctor noted that it was appropriate for her second trip to be in her relative past. (TV: The Unquiet Dead)
Despite initially accepting that he would take Martha Jones on "just one trip" in the TARDIS, the Tenth Doctor "stretch[ed] the definition" to allow for a trip to her relative past as well as a trip to her relative future. (TV: Gridlock)
In River Song's World, Charles Dickens promoted his "new Christmas special" on television as involving ghosts and the past, the present and future, all at the same time. (TV: The Wedding of River Song)
While attempting to flee from the sentient oil with Nardole and Bill Potts, the Twelfth Doctor took the TARDIS to the Dalek-Movellan War, which he explained was "the past" to Bill, a native of 2017. (TV: The Pilot)