The letter X stands for the unknown or the mysterious: Project X; the X-Men; the X-Files. When physicist Wilhelm Röntgen discovered a baffling new form of radiation in 1895, naturally he named them “X-Rays”. We absorbed these cultural connotations long before we encountered X in maths class, where we learned to solve algebraic equations for X. But why not some other letter? Why does X always stand for the unknown?
Click here to read the full article at Cosmos Magazine.