- 1. (π)a very famous mathematical unit. The circumference of a circle is equal to pi multiplied by the diameter, so pi is equal to the ratio between the circumference and diameter of a circle, any circle. It turns out that pi is an irrational number, which means that its decimal expansion is nonterminating and nonrepeating. To 25 significant digits, pi equals 3.141 592 653 589 793 238 462 643. The Swiss mathematician Leonhard Euler (1707-1783) adopted the lower case Greek letter π (pi) for this ratio in 1737; although he was not the first to use it, he popularized it through his many mathematical writings. Pi sometimes appears to be a unit of angle measure equal to π radians or 180°. This is because mathematicians regard angle measurement as dimensionless (length divided by length) and therefore omit the unit "radians".2. see pica
Dictionary of units of measurement. 2015.