A timeline was a method of ordering chronology, creating a history for an individual, place, event, etc.
The term "timeline" was sometimes used interchangeably with other temporal terminology like "time stream." (AUDIO: Reborn) On two occasions, the Seventh Doctor pondered that Daleks would think twice before doing something to alter the timeline (TV: Remembrance of the Daleks) and the timestream. (PROSE: Transit) The Eighth Doctor later stated that Time Roaches couldn't be let out of the time vortex into the timeline. (AUDIO: Foreshadowing)
Sometimes reference would be made to "the timelines," multiple rather than singular. The Master and the Daleks manipulated and shifted the timelines to conquer Earth in 1921 (AUDIO: Master of the Daleks) and the Sanukuma described the Last Great Time War as being raged across the timelines. (AUDIO: The Rulers of the Universe) During the war, the Daleks developed the annihilator weapon which could erase a whole species without affecting the timelines, meaning they would still be remembered even though they'd never existed. (AUDIO: Legion of the Lost)
When dealing with the interference of the Star Men, the Fifth Doctor mentioned that they were damaging the "timeline," the "timelines," and that they had inserted themselves into "this reality's time stream." (AUDIO: The Star Men)
Centering on what the Time Lords called the "primary timeline," (AUDIO: Reborn) alternate timelines could diverge, which if left unchecked would end the real universe millions of years before it was meant to due to there being only a finite amount of matter and energy in the uni-verse. (PROSE: Blood Heat)
Universal History Edit
The timeline of the universe spanned billions of years. However, most of the Doctor's involvement seemed to be in what was sometimes called the Humanian Era, that part of history which encompassed human existence. As a result, the Doctor's TARDIS was most often seen between the time humans first evolved through to about the 82nd century. Very occasionally the Doctor went further than the 85th century, into what might be called the "far future".
The Doctor was known to have visited, or at least been affected by, people and places in the following broad divisions of time as measured by at least one human dating system: