Oughtred, William (1575–1660)
William Oughtred was an English clergyman and mathematician who invented the slide rule and who, in the first English edition of his Clavis mathematicae in 1647, first used the name 'pi' for the number 3.141... He wrote it π.δ, where π stood for the English word "periphery" (what we would call circumference), the dot was his symbol for division, and δ stood for the English word 'diameter.'
The use of π to represent words starting with the letter 'p,' like 'periphery,' was not uncommon. Before π = 3.14... caught on, π was variously used to indicate a point, a polygon, a positive number, a power, a proportion, the number of primes in a series, and a factorial (which is a product).
Oughtred used the same notation in all later English and Latin editions of his book, but not in the earlier first Latin edition. He was a prodigious inventor of mathematical symbols, though most of them have not survived. He did, however, introduce a couple of other symbols we still use: × for multiplication (as distinct from the letter x, which he also used for this purpose) and ± for 'plus or minus.'
Oughtred served as Rector of Albury from 1608 or 1610 until his death. Here he tutored many young mathematicians of the time, including John Wallis, Ward, Moore, Scarburgh, and Christopher Wren. Moreover, all English mathematicians for the next century, including Isaac Newton, learned algebra from Clavis mathematicae (first published in 1631).