The jansky (Jy) is a unit of spectral flux density used in radio astronomy, equivalent to 10-26 watts per square meter per hertz, to describe the brightness of sources at particular frequencies. It is neither a CGS nor an SI unit, but is devised by/for radio astronomers, derived from standard units, consisting of a unit much closer to the order-of-magnitude encountered in radio astronomy. Examples:
Commonly seen used are millijansky (mJy, also referred to as m.f.u. for milli flux unit), and megajansky (MJy).
The jansky is useful to characterize a point source: if a single source is within the beam, spectral flux density offers a measure that other radio telescopes can reproduce. For extended sources, "Jansky per solid angle" has been used in radio astronomy, as have other measures of specific intensity, such as brightness temperature.