Vertical and horizontal (radio propagation)
In radio propagation, horizontal plane is used to plot an antenna's relative field strength in relation to the ground (which directly affects a station's coverage area) on a polar graph. Normally the maximum of 1.000 or 0 dB is at the top, which is labeled 0° running clockwise back around to the top at 360°.
Source: Wikipedia — Vertical and horizontal (radio propagation) (CC BY-SA 4.0)