The ratio of the circumference of a circle to its diameter is an irrational number, so there's no way to express its decimal digits explicitly without using an approximation: 3.14159 . . . . Hence, it's handy to have a special symbol to represent the number in all its glory.
Welsh mathematician William Jones (1675-1749) was apparently the first one, in 1706, to use the Greek letter π (pi) in connection with this number. Jones was influential, a friend of Isaac Newton (1643-1727) and Edmond Halley (1656-1742), and later an official of the Royal Society. Leonhard Euler (1707-1783) adopted the practice in 1737, and π became a staple of the mathematical literature.
The number and its symbol have now achieved such renown that Pi Day is celebrated on March 14 (3/14) in more and more places each year. And puns associated with π, pi, and pie abound.
It’s probably not surprising to see members of a student math club wear T-shirts referring to pi.
Student T-shirt from Southwest Texas Junior College.
Math club T-shirt at Winona State University, Minnesota.
But you could also argue that familiarity with pi in the mathematical sense has contributed to the use of pi and its symbol in other contexts, such as restaurant signs and menu items.
In New Orleans:
Pie, Pizza & Pastas is in the Warehouse District of New Orleans.
In Austin, Texas:
Featured at Amy's Ice Creams in Austin, this flavor is delicious!
In Washington, D.C.:
Digits of pi have also figured into a subway mural in Toronto (see "Sliding Pi in Toronto") and in the design of math jewelry, including an apple pi necklace.
Photos by I. Peterson