Imaginary number

An imaginary number is a complex number whose real part is equal to 0. In the complex plane, these numbers lie on the imaginary axis. They are sometimes also refered to as pure imaginary numbers,

The complex number $z$ is imaginary if and only if $z = i \textrm{Im}(z)$ , where $i = \sqrt{-1}$ is the imaginary unit and $\textrm{Im}$ is the imaginary part function.