Π


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

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)