In mathematics, a variable is a symbol, typically a letter, that holds a place for constants, often numbers. In 1637, René Descartes "invented the convention of representing unknowns in equations by x, y, and z, and knowns by a, b, and c". Descartes' convention is still commonly in use. The history of the letter x in math was discussed in an 1887 Scientific American article.

Source: Wikipedia