The area that a flash will illuminate is also known as the angle of coverage.
It is obviously important that it is greater than, or equal to, the angle that your lens can ‘see’.
Technical
The angle of the light-cone or pyramid emanating from the light source.
The quoted angle of coverage for a flash unit corresponds to the angle between two rays, on opposite sides of the beam axis, chosen so that the light intensity is half of that at the beam center. i.e. it is assumed that the useful field of illumination ends at the point where the light intensity has fallen by 1 EV (1-stop) relative to the center.
Circular light sources are given a single value for angle of coverage; rectangular light sources are given two values (Horizontal and Vertical).