Greek letters have been part of mathematics for a long time. Ancient Greek mathematicians used letters to label geometric points and represent numbers, but many of the meanings familiar today developed centuries later. Take π. William Jones used it for the ratio of a circle’s circumference to its diameter in 1706. Leonhard Euler later helped make the symbol widely known. There is no universal rule that gives each letter just one meaning. A physicist might use μ for friction, while a statistician uses it for a population mean. The context tells you what the symbol represents.