Imaginary numbers

In mathematics, the notion of number has been extended over the centuries to include zero, negative numbers, rational numbers (such as $\frac{1}{2}$), real numbers (such as $\sqrt{2}$ and $\pi$) and complex numbers, which extend the real numbers by introducing imaginary numbers (an imaginary number consists of the real part and the imaginary part; a common imaginary number, $i$, is defined as the square root of minus one, $i = \sqrt{-1}$).