A radial is a line drawn along a vector.
A radial pattern is one that appears to radiate from a point, like the spokes from the hub of a wheel.
In radio, evenly-spaced points along evenly-spaced radials on a map are used to determine average elevation above mean sea level (AMSL) within a radio station's coverage area (including broadcast stations and mobile phone towers, among others). This in turn determines height above average terrain (HAAT), which greatly affects a station's range (more so than power), and potential for interference with other stations.
Stations at low frequencies like the AM broadcast band also require grounding or earthing wires to be buried in the soil in a radial pattern, in order to have a proper ground plane. These wires are also called radials, ground radials, grounding radials, or earthing radials.
See also: radial tire, axial