Cambridge Dictionaries online Cambridge Dictionaries online

The most popular online dictionary and thesaurus for learners of English


English definition of “pi”


noun [U] (also π)     /paɪ/
a Greek letter, especially used in mathematics as a symbol for the number (approximately 3.14) used to calculate the size of circlesLetters of alphabetsLettering and alphabetsMathematical symbols
(Definition of pi noun from the Cambridge Advanced Learner's Dictionary & Thesaurus © Cambridge University Press)
Focus on the pronunciation of pi

Definitions of “pi” in other dictionaries

Word of the Day


physical activity that you do to make your body strong and healthy

Word of the Day


Read our blog about how the English language behaves.

Learn More

New Words

Find words and meanings that have just started to be used in English, and let us know what you think of them.

Learn More