Silence

Silence was the absence of sound.

On Xeros, Barbara Wright was the first to notice the silence, describing it as "the sort of silence you can almost hear." Ian Chesterton likened it to that of a graveyard. The silence was the result of the TARDIS having jumped a time track. (TV: The Space Museum)

On a doomed planet, the First Doctor was the first to notice the silence, which reminded him of Xeros. In this case, the silence was due to the complete lack of animal life. (TV: Galaxy 4)

Silence was the cause which the Church of the Papal Mainframe, led by Mother Superious Tasha Lem, dedicated itself to at the start of the Siege of Trenzalore, becoming the Church of the Silence; among their ranks were a species known as the Silents, who served as confessional priests. The Church was reformed as such to ensure that the Eleventh Doctor would not speak his name and thus answer the Question sent by the Time Lords from the crack in time on the planet Trenzalore; were he to do so, the Time Lords would return to the universe, resulting in a new Time War with the Daleks and the many other races waiting above Trenzalore. Realising this, the Doctor indeed never spoke his name, and so the Siege ultimately ended with the Time Lords remaining in their pocket universe. (TV: The Time of the Doctor) Eventually, Gallifrey returned to the universe in a more discrete manner, at the end of the universe. (TV: Hell Bent)