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)