Circle

The Greek letter pi was, according to both Tegan and the First Doctor, the ratio of any circle's circumference to its diameter. This was considered to be "basic mathematics". (TV: The Five Doctors)

The Twelfth Doctor used pi to communicate with the Boneless, as the basic mathematics was the only common thing he could find with the creatures from another dimension, reasoning that even in a flat world that would have circles. (TV: Flatline)

When brought before the Dalek Eternity Circle, the War Doctor noted that the Daleks were arranged in a hexagonal configuration instead of a circle, leading him to question the group's name. (PROSE: Engines of War)