Equations began with Diophantus: he was the first to think of naming the unknown in a problem. This was no trivial idea, since naming something is a way of taming it, of giving it a reality. Once named, the unknown could be used in calculations. Diophantus called it arithmos, meaning "the number" in Greek. Everyone knows this term, from which the word "arithmetic" was coined. He then wrote out the resulting relations in full. This produced cumbersome sentences, though they remained clear.
Diophantus's tradition was taken up by medieval Arab mathematicians, who used a different term. In the 9th century, al-Khwarizmi referred to the unknown as shay, meaning "the thing." The Italian algebraists of the Renaissance used the same word, cosa in Italian. The Andalusians, then under Arab influence, wrote the word in Latin characters as xay.
