pi

Concise Oxford English Dictionary © 2008 Oxford University Press:
pi1 /pʌɪ/

▶noun

• 1 the sixteenth letter of the Greek alphabet (Π, π), transliterated as ‘p’.

• 2 the numerical value of the ratio of the circumference of a circle to its diameter (approximately 3.14159).

• 3 [as modifier] Chemistry & Physics (of an electron or orbital) with one unit of angular momentum about an internuclear axis.

▶symbol (Π) mathematical product.

– origin from Gk: the numerical use comes from the initial letter of Gk periphereia ‘circumference’.

Concise Oxford English Dictionary © 2008 Oxford University Press:
pi2 /pʌɪ/

▶adjective Brit. informal short for pious.