Fixed point in time

Flux points in Time were all small points in time at which events could be changed.

Definition
Flux points were insignificant events that could be altered with no consequence. Due to this, the Doctor normally meddled with flux points in time. (DW: The Christmas Invasion, NSA: I am a Dalek) When the Doctor first met Martha Jones, he told her that 'Crossing into established events is strictly forbidden ... except for cheap tricks.'. (DW: Smith and Jones)

Altering flux points
1969 was in flux, as discovered by the Doctor when it was revealed that WOTAN was supposed to conquer Earth (PDA: The Time Travellers) instead of being defeated by the Doctor. (DW: The War Machines)

Sarah Jane Smith was shown an alternative timeline by the Doctor, explaining how 1911 was now in flux because of the threat of Sutekh. (DW: Pyramids of Mars)

As the Doctor explained to Rose Tyler, when she said that he couldn't give dead Human corpses to the Gelth, as she knew for a fact that dead bodies weren't walking around in 1869, time was in flux, changing every second and her "cosy little world can be rewritten like that". (DW: The Unquiet Dead)

Although the Doctor noted to Rose that Harriet Jones would serve three successive terms as British Prime Minister, he himself would later, in his next incarnation, cause her political downfall, leading to her losing the title of Prime Minister to Harold Saxon. (DW: The Sound of Drums) She would later die the following year at the hands of the Daleks during their invasion of Earth. (DW: The Stolen Earth)

After they saw Frank Openshaw exterminated by a Dalek, Rose Tyler and the Doctor meddled in history for him so that he met his wife Sandra years earlier than in the original timeline. (NSA: I am a Dalek)

In 2020, when a drilling operation in Cwmtaff disturbed a Silurian civilisation, the Doctor told Amy Pond, Nasreen Chaudhry and Eldane that this encounter could lead to either a peaceful relationship or a devastating war. The Doctor described these events as opportunities. (DW: Cold Blood)

When the Dalek Emperor and his flagship survived the end of the Time war in 199,909, the Daleks secretly altered the history of the Fourth Great and Bountiful Human Empire, controlling it via the Jagrafess. Unlike what the Ninth Doctor remembered it to be, the Human race at its height, humanity's development was instead stunted. (DW: The Long Game,Bad Wolf)

At one point, the Korven altered the history of Earth (which they previously invaded in 2480), causing part of it to be controlled by the corrupt, totalitarian Department. (K9TV: The Eclipse of the Korven)