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This is Greek letter pi, which is used to denote the number pi.

This is Greek letter pi, which is used to denote the number pi.

Todd Helmenstine
Question: What Is Pi Day?
Answer: March 14 (03/14) is Pi Day, in honor of the number that is referred to as pi. Pi is the ratio of a circle's circumference to its diameter, which comes out to be 3.14159... (The calculation has been carried out past a trillion digits so far). Pi is an important mathematical constant, so a day was designated to honor it.

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