What property of a radio wave defines its polarization?

In radio communications, the polarization of an electromagnetic wave is conventionally defined by the orientation of its electric field relative to the Earth's surface. For example, a vertically mounted antenna emits a radio wave with a vertical electric field, resulting in vertical polarization. While the magnetic field is also present and oriented at right angles to the electric field, it is not used to define the wave's polarization. Distractors mentioning ratios of energy or velocity are entirely unrelated to the physical alignment of the wave in space.