In mathematics, an imaginary number is a number whose square is negative. The term was coined by René Descartes in the seventeenth century and was meant to be derogatory: obviously such numbers don't exist. Nowadays we find the imaginary numbers on the vertical axis of the complex number plane. Every imaginary number can be written as where is a real number and the imaginary unit with the property that
Despite their name, imaginary numbers are just as real as real numbers; see Complex number#Definition on how they can be constructed.