Π

The Greek letter pi (3.1415965...) 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 mathematic 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)