In radio astronomy, the term flux density is used for the power from a radio source received by the radio telescope, per unit area of the receiver (e.g., of the reflector of a dish telescope), as watts per meter squared.
Often the phrase flux density is short for spectral flux density, i.e., the flux density per unit of frequency (hertz). The jansky is a unit of this quantity used in radio astronomy. Given these two scalings, measurements by two separate well-calibrated radio telescopes of a source should match.
The way in which a measured flux density relates to the source depends upon whether it is a point source or an extended source that exceeds the radio telescope's beam: whether it represents all power received from that source (at a given frequency) or that from a portion of it. In the latter case, a conversion would be done to spectral radiance (sometimes called spectral intensity or spectral brightness).
While this term is used in radio astronomy, there are other terms for the equivalent measure that are used in other branches of astronomy and other areas where electromagnetic radiation is measured, for example spectral irradiance.