The spectral flux density (S) is a measure of the rate of electromagnetic radiation (EMR) power passing through a unit area at a particular wavelength. It is commonly used in radio astronomy to characterize the power received per unit area of the antenna from an entire radio source. Thus, from point sources, the spectral flux density received by different radio telescopes does not depend upon their different collecting areas. The spectral flux density may be cited in terms of either frequency or wavelength: the common unit in astronomy, the jansky is by frequency.
A telescope's spectral flux densities from a radio source, as a function of wavelength (or frequency) are effectively a distribution function: a function only yielding a non-zero power when integrated over an interval of wavelengths. Since wavelength and frequency are reciprocals, the spectral flux density value according to each differs, the difference increasing toward the longest or shortest wavelengths.
The term flux density is used similarly but for a (possibly infinite) range of wavelengths rather than one specific wavelength. It may be used to refer to all wavelengths detectable by a particular receiver or through a particular filter. Note that spectral flux density is often shortened to flux density and both are sometimes shortened to flux, so paying attention to how the terms are being used is useful. The meaning of the term spectral flux density is rather clear, given the meaning of the individual words, but in the many branches of technology (and astronomy), terms regarding the various measures of the strength of EMR vary, sometimes ambiguously.
For characterizing the brightness of portions of or points within a resolved (extended) source, the term surface brightness is typically used, which is effectively flux density per region of the sky.