The Time Brain was a giant brain created by the Rani. By using the minds of geniuses from across time and space, the Rani planned to use the brain to do the calculations needed to create a Time Manipulator. Her plans were stopped when the Doctor's brain was added, stopping the Rani from controlling it. (DW: Time and the Rani)
Individuals used to create the Time Brain included Albert Einstein, Louis Pasteur, and Hypatia.