Pi is the sixteenth letter of the Greek alphabet and the abbreviation used to name a relationship, the ratio between the circumference of a circle and that circle’s diameter. As such, it is a mathematical constant. Pi allows you to find the circumference of the circle if you know the radius or diameter, and allows you to find the radius and diameter if you only know the circumference. The Greek form, π, is often used in mathematics and scientific settings.

Pi is given as different figures, both for ease and because the calculation of pi has improved over time. In middle school or junior high, when it is first introduced to students, a value of 3.14 may be used. As students mature, they may be given the value 3.14159, and then 3.14159265. The fact is that pi had been calculated to 707 places in the nineteenth century, and in the twenty-first century, we can claim that is has been calculated to 200 billion digits.

Pi is an irrational number, which means that it is a real number with a nonrepeating decimal expansion: it cannot be represented by an integer ratio. It was Johann Lambert ,a German mathematician, physicist, and astronomer after whom the unit of brightness, the lambert, was named, who proved the irrationality of pi in 1770.

In 1882, British physicist Ferdinand Lindemann showed that π is a transcendental number, which means that it cannot be the root of any algebraic equation with rational coefficients. The problem of trying to construct a square that has equal area to a given circle had been proposed in 1800 B.C., and Lindemann used his discovery to prove what had been suspected: that “squaring the circle” is impossible using only straight edge and a compass.

As an acronym, PI can also refer to a number of other concepts. On the list are: private investigator, politically incorrect, protease inhibitor, polynomial identity, and many other terms.