An imaginary number is a number whose square is negative. Every imaginary number can be written in the form ib where b is a real number and i is the imaginary unit with the property that i 2 = -1. For example, √-16 is an imaginary number, since √-16 = √-1.√16 can be written as 4i.
Imaginary numbers are complex numbers in which the real part is zero. The term "imaginary" is unfortunate because it suggests something that has less reality than a "real" number, which isn't the case.