Fixed point in time

Flux points in Time were all small points in time at which events could be changed in the post-Gallifrey universe.

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)

Among all the insignificant details in history were the fixed points in Time, highly important events and/or people which could not be changed. (DW: The Fires of Pompeii, The Waters of Mars)

Flux points in time possibly didn't exist before Gallifrey's destruction in the Last Great Time War, as the Eye of Harmony was stated to "balance all things" so they would neither flux "nor change their state in any measure". (DW: The Deadly Assassin)

Altering flux points
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 Ninth 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 in May 2008. (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 have led 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 Doctor remembered it to be, being the Human race at it's height, Humanity's development was stunted. (DW: The Long Game,Bad Wolf)

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: Eclipse of the Korven)