It is called a solid angle. 2-dimensions it is a patch on a sphere. The source's flux is the measure of the power received per unit area from the astronomical source, integrated over all frequencies. Luminosity is the surface area of the sphere surrounding the source with a radius equal to the distance multiplied by the flux of the source. Since most astronomical sources produce emission over a broader wavelength range than any one detector can detect, flux is not well suited for accurate radio measurements.
Flux density is the flux per unit frequency interval.
The Jansky is used often to measure the flux density at radio, infrared, and optical wavelengths. The flux is simply the integral of the flux density over all frequencies.