1. (symbol S) Spectral flux density, a quantity that indicates the observed strength of an astronomical source. It is a measure of the radiant power passing through unit area in unit interval of frequency or wavelength. The SI unit is W/m² per Hz, but the special unit jansky is more common in radio or infrared astronomy. In infrared astronomy, the unit W/m² per μ‎m is often used. The term is occasionally (and more correctly) used to mean radiant power per unit area (W/m²).

2. (symbol B) Magnetic flux density, a quantity that indicates the strength and direction of a magnetic field. The SI unit is the tesla.