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1The sixteenth letter of the Greek alphabet (Π, π), transliterated as ‘p’.
‘In the Greek alphabet, Lambda, Pi, and Eta are the first letters of the words logos, pathos, and ethos, which are listed by Aristotle in The Rhetoric as the three modes of persuasion.’
‘The western Greek alphabet changed pi into a backwards loop shape.’
‘Pi is the sixteenth letter of the Greek alphabet. In the system of Greek numerals it has a value of 80.’
‘The 16th letter of the Greek alphabet, Pi is an internationally recognized symbol evoking images of circles and spheres.’
‘This is true of no other letter of the alphabet except pi, and here the name is a natural development of the Semitic name pe.’
1.1The numerical value of the ratio of the circumference of a circle to its diameter (approximately 3.14159).
‘So far, pi has been calculated to 1.24 trillion decimal places with the aid of a supercomputer.’
‘If one knows that the area of the circle equals pi times the square of the radius of the circle then one can also compute pi from that relationship.’
‘Saying that pi is 3.14 without addressing the fact that it goes on infinitely afterward overlooks the true essence of the number.’
‘The circumference of a circle can be found by multiplying pi (which is equal to 3.14) by the diameter of the circle.’
‘Pi is the constant created by the relationship of every diameter to its circle.’
1.2Astronomy [followed by Latin genitive]The sixteenth star in a constellation:
‘Pi Herculis’
‘Pi Herculis is particularly noticeable for its reddish complexion and is an excellent place to begin studying the constellation.’
‘Lower letters are generally assigned to brighter stars, but a star designated pi is not necessarily the 16th brightest star of the constellation (for example, pi Puppis is number two in brightness in the constellation Puppis).’
‘The red giant Pi Leonis in the constellation of Leo the Lion has a radius 80 times larger than our own Sun.’
1.3Physics Chemistry [as modifier]Relating to or denoting an electron or orbital with one unit of angular momentum about an internuclear axis:
‘the π orbital system of the benzene molecule’
‘The bonding pi orbital is lower in energy than the atomic p orbitals, whereas the antibonding pi orbital is higher in energy than the atomic pi orbitals.’
‘Both of the electrons in the pi bond are found in the pi bonding orbital.’
‘The result is that the overall hybridization creates a continuous pi orbital that encompasses the entire benzene ring.’
‘The lowest pi orbital of benzene is called the A2u orbital.’