In the earliest usages, the Greek letter was used to denote the semiperimeter (semiperipheria in Latin) of a circle and was combined in ratios with (for diameter or semidiameter) or (for radius) to form circle constants.See p. 220: William Oughtred used the letter to represent the periphery (that is, the circumference) of a circle. (Before then, mathematicians sometimes used letters such as or instead.) The first recorded use is Oughtred's , to express the ratio of periphery and diameter in the 1647 and later editions of .See, for example, (English translation: ) Barrow likewise used to represent the constant , while Gregory instead used to represent .
In the earliest usages, the Greek letter was used to denote the semiperimeter (semiperipheria in Latin) of a circle and was combined in ratios with (for diameter or semidiameter) or (for radius) to form circle constants.See p 220: William Oughtred used the letter to represent the periphery (that is, the circumference) of a circle. (Before then mathematicians sometimes used letters such as or instead.) The first recorded use is Oughtred's , to express the ratio of periphery and diameter in the 1647 and later editions of .See, for example, (English translation: ) Barrow likewise used to represent the constant , while Gregory instead used to represent .