Jones's Use of the Symbol π
π gets its symbol
The Greek letter π (pi) was originally used as an abbreviation for the Greek word for "periphery" and combined with the Greek letter δ (delta; for diameter) or ρ (rho; for radius) to indicate relevant dimensions of a circle. William Oughtred made the first known use of such a notation (π/δ) to indicate the ratio of circumference to diameter in 1647, and Barrow and Gregory used similar notations (π/δ for 3.14… and π/ρ for 6.28, respectively). The first known use of the symbol π on its own to mean 3.14… was published by William Jones in 1706. This notation was universally adopted following advocacy by Euler, though disagreement on the convention of π = 3.14… vs. 6.28… persisted at least until 1761.
Self-taught Welsh mathematician William Jones published Synopsis Palmariorum Matheseos (A New Introduction to the Mathematics) in 1706. Page 263 of that work contains the earliest standalone use of the symbol π to denote the ratio of a circle's circumference to its diameter 3.14…. Interestingly, page 243 uses the same symbol to refer to twice that value, or 6.28…, so it appears Jones suffered from the same difficulty in deciding between those two conventions that persisted for more than 50 years subsequently.