Pi is a mathematical variable.
In probability , and statistics , a multivariate random variable or random vector is a list of mathematical variables each of whose value is unknown , either because the value has not yet occurred or because there is imperfect knowledge of its value . The number is a mathematical constant , the ratio of a circle 's circumference to its diameter , commonly approximated as 3.14159 . The ubiquity of makes it one of the most widely known mathematical constants both inside and outside the scientific community ; several books devoted to it have been published , the number is celebrated on Pi Day , and record-setting calculations of the digits of often result in news headlines .