Upper half-plane

In mathematics, the upper half-plane H is the set of complex numbers with positive imaginary part:

The term arises from a common visualization of the complex number x + iy as the point (x,y) in the plane endowed with Cartesian coordinates. When the Y-axis is oriented vertically, the "upper half-plane" corresponds to the region above the X-axis and thus complex numbers for which y > 0.