Add Yahoo as a preferred source to see more of our stories on Google. A planet surrounded by the number pi with the first 15 decimal places highlighted. Pi is an irrational number, meaning it has an ...

Pi, a mathematical constant denoted by the Greek letter π, is the ratio of a circle's circumference C to its diameter d: π = C/d. The circumference of a circle is, in turn, equal to 2πr, where r is ...