Pi (π) is the ratio of the circumference of a circle to its diameter. It is a mathematical constant that appears in many formulae across mathematics and physics. The value of Pi is approximately equal to 3.14159.
Pi is an irrational number, which means that it is a real number that cannot be expressed by a simple fraction. That's because pi is what mathematicians call an "infinite decimal" — after the decimal point, the digits go on forever and ever.
Here's an example: If you have a circle with a diameter of 1 meter, then the circumference of the circle is Pi times the diameter which is approximately 3.14159 meters.
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