Complex plane
From AoPSWiki
The complex plane is one representation of the complex numbers. It is a coordinate plane with two perpendicular axes, the real axis (typically plotted as the horizontal axis) and the imaginary axis (typically plotted as the vertical axis). Any complex number 
can be plotted on it, with the real part 
as the real (horizontal) coordinate and the imaginary part 
as the imaginary (vertical) coordinate. The intersection of the two axes (the origin of the coordinate system) corresponds to the complex number 0, while a point two units to the right and one unit down from the origin corresponds to the complex number 
.
