What is the meaning of π?

Pi, an irrational constant representing the ratio of the circumference of a circle to its diameter; approximately 3.14159265.

pion, pi meson

homotopy group

prime-counting function

A voiced labial click consonant (IPA [ɡ͜ʘ]).

A voiceless labiodental plosive ().

Lower-case pi (πεῖ), the 16th letter of the ancient Greek alphabet. It represented the voiceless unaspirated bilabial plosive /p/. It is preceded by ο and followed by ρ.

The lower case letter pi (πι), the 16th letter of the modern Greek alphabet.

Source: wiktionary.org