A mathematics teacher named William Jones is credited with first using the symbol π in 1706 to represent the concept of the ratio between a circle's circumference and diameter. It had previously been ill-defined.
Prior to Jones' usage of the π symbol, the ratio of a circle's circumference and diameter was known as "the quantity which, when the diameter is multiplied by it, yields the circumference." William Jones first used the symbol in his book, "Synopsis Palmariorum Matheseos, or A New Introduction to the Mathematics." Jones believed, but could not prove, that pi was irrational. In 1761, Johann Lambert first proved that pi was an irrational number.