Pi was known to the ancient Egyptians and Babylonians some 4,000 - 5,000 years ago.
The constant p (Greek letter pi) is, classically, defined as the ratio of the circumference p of a circle to its diameter d:
p = pd = 2pr
and, as proved by Archimedes of Syracuse (287-212 BC) in his famous Measurement of a Circle, the same constant is also the ratio of the area A enclosed by the circle to the square of its radius r:
A = pr^2.
