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. Examples:
Commonly seen used are millijansky (mJy, also referred to as m.f.u. for milli flux unit), and megajansky (MJy).