Pi (3.1415926535...) is the ratio of a circle's circumference to its diameter. (SG1: "Thor's Chariot") (SGA: "Tabula Rasa")
Dr. Rodney McKay knows Pi to at least 12 decimal places and was easily able to remember them on the first day he was diagnosed with an ailment known as Second Childhood. However, after only two weeks, he barely remembered it to just two decimal places. (SGA: "The Shrine")
At some point after his arrival on Destiny, Eli Wallace set Destiny's computer to calculate pi, most probably to the last digit. (SGU: "Resurgence")