pi

The word 'pi' entered English in the early 18th century directly from the Greek letter name 'πῖ'. This Greek term was itself borrowed from the Phoenician alphabet, where the letter 'pē' meant 'mouth', likely due to the shape of the original pictogram. The application of the Greek letter π to represent the mathematical constant was first proposed by Welsh mathematician William Jones in 1706, popularized later by Leonhard Euler. This usage stems from 'π' being the first letter of the Greek word 'περιφέρεια' (periphery), relating to the circumference of a circle, thus creating a lasting symbolic link between an ancient alphabet and a fundamental geometric ratio.

This is a simplex loanword from the Greek alphabet. The letter's name, 'pi', is derived from the Phoenician letter 'pē', meaning 'mouth'. Its function in English is as a noun representing the specific Greek letter and its associated mathematical constant.