i is defined as a number which when multiplied by itself gives -1. In 1545, Cardano decided to invent a number which when multiplied by itself gave -1 as a product. Since this number didn't seem to exist in the real world, but only in imagination, he called this number an "imaginary number." In 1777, leonhard Euler symbolized the square root of -1 as i(for "imaginary")






Illustration: i=






Reference: Realm of Algebra