# 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, $$i = \sqrt{-1}$$).
