Who, in 1706, first gave the greek letter "pi" its current mathematical definition? * ?

The notation with the Greek letter π comes from the initial words of Greek origin περιφέρεια 'periphery' and περίμετρον 'perimeter' of a circle, notation that was first used by William Oughtred (1574-1660) and whose use was proposed by the Welsh mathematician William Jones (1675-1749); although it was the mathematician Leonhard Euler, with his work Introduction to the infinitesimal calculation, of 1748, who popularized it.

It was previously known as Ludolph's constant (in honor of the mathematician Ludolph van Ceulen) or as a constant of Archimedes (not to be confused with Archimedes' number).

Jones poses the name and symbol of this number in 1706 and Euler begins to spread it in 1736.

Answer:

Welsh mathematician William Jones