Word of the Day


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Liz Potter
Written by Liz Potter


in mathematics, a number written as the symbol π that is used to show the relationship between the distance around a circle and the distance across it

View the full definition in the Macmillan Dictionary.

Origin and usage

The noun pi, which is the name of the 16th letter of the Greek alphabet, was borrowed into English from Latin. Its use as a mathematical symbol π by the Welsh mathematician William Jones dates from the early 18th century.


March 14 is pi day, an annual celebration of the number pi, the mathematical constant that shows the ratio between the radius of a circle and its circumference. The day has been celebrated since 1988 and the date 3/14 was chosen because 3, 1 and 4 are the first three digits of pi. Of course this only works if you write the date in the American manner, with the month before the day; the British way of writing it would be 14/3. Pi day celebrations generally feature pie, because who doesn’t love a pun, but also because most pies are round and thus resemble the circle that pi relates to. ‘Pie’ appears in many compounds and fixed phrases, which you can explore by looking at the ‘other entries’ box to the right of the entry. To pick just a few of them: a pie chart is a circular chart divided into sections, while to be as nice as pie is to behave very pleasantly, especially if this was unexpected. Pie in the sky is something good that will never happen, while to have a finger in every pie is to be involved in many different things.


Pi is not merely the ubiquitous factor in high school geometry problems; it is stitched across the whole tapestry of mathematics, not just geometry’s little corner of it.
(Robert Kanigel, biographer and science writer)

Related words

base, constant, digit, integer

Browse related words in the Macmillan Thesaurus.

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Liz Potter

Liz Potter

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