Π


 * One of the Crimson Hand members was known as Pi.

The mathematical concept of Pi (3.14159...) was the ratio of any circle's circumference to its diameter.

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. (DW: The Five Doctors)

The square root of Pi is (approximately) 1.772453850905516027298167483341. The Tenth Doctor used that sequence to test the intelligence and memory retention of the Midnight entity. (DW: Midnight)