Pi (noun)

The symbol for the ratio of the circumference of a circle to its diameter, approximately 3.14159.

Origin:

Mid 16th century: symbol π, representing the ratio of the circumference of a circle to its diameter, introduced by the greek mathematician archimedes (3rd century bc).

Examples:

1. The value of pi is approximately 3.14159.
2. The circumference of the circle is pi times the diameter.
3. Pi is an irrational number, meaning it can't be expressed as a simple fraction.
4. He memorized the first 100 digits of pi.
5. The pi day is celebrated on March 14th (3/14).