Π

The Greek letter π (3.14159265...) 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)

In the Tomb of Rassilon on Gallifrey a death trap existed to guard Rassilon. Stepping far enough into a particular room would cause lightning to strike down upon the intruders. Pi was the only way to reach across the room without being destroyed. (TV: The Five Doctors)

The square root of pi was 1.772453850905516027298167483341. The Tenth Doctor used that sequence to test the intelligence and memory retention of the Midnight entity. (TV: Midnight) The Twelfth Doctor similarly 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. (TV: Flatline)

The Omniverbum was similar to pi in that both were transcendental and could only be approximated. (AUDIO: ...ish)