The Greek letter 𝜋 ("Pi", pronounced - at least in the UK - like "pie") when
encountered in mathematics is usually used as shorthand for a pretty important
number. This number can be defined in various ways, but arguably the simplest is
"the ratio of a circle's circumference to its diameter." To put it another way, 𝜋 tells you how many times bigger the circumference is than the diameter; or it's the number you get if you divide a circle's circumference by its diameter:

\[ \pi = \frac{C}{d} \]

One way that 𝜋 could be calculated is by measuring the diameter and circumference of a given circle and dividing the latter by the former. But one of the reasons we might want to know a value for 𝜋 in the first place is that it is difficult to measure the circumference of a circle.