Ancestral memory, also known as race memory, was a memory of racial history present at birth in an individual. This resulted from an event repeated enough times that it was incorporated into a species' genome.
Humanity had race memories of transcendental beings like Kronos (TV: The Time Monster) and the Great Old Ones, preserved as nightmarish creatures of chaos. (AUDIO: The Roof of the World) The memory of them or they themselves could be reached using mind-expanding drugs. (PROSE: White Darkness) Some humans also had an ancestral fear of Silurians, causing mental breakdowns. (TV: Doctor Who and the Silurians)
The Tractators once invaded Trion, causing future Trions to be terrified of them. Turlough went into shock after seeing Tractators as they awakened the ancestral memory within him. This allowed him to remember how the Trions had previously defeated the Tractators by separating them from the Gravis. (TV: Frontios)
A Plot by the Prometheans involved using the Ancestral Memory of the image of Sun to revert the minds of those on Earth back to the Stone Age. The Doctor was able to reverse this by sabotaging the Chameleon Circuit in his Tardis, so that the Blue Box image would be implanted into the Ancestral Memory of Earth's History. (COMIC: Hunters of the Burning Stone)