The term luminosity is used with different meanings. In a astronomy, it can mean the electromagnetic power radiated by an object such as a star and may be specified with units of watts, or alternatively relative to the emission of the Sun (solar luminosities). If radiation from all spectral region is taken into account, that quantity would more specifically called the bolometric luminosity. One may also consider only radiation in certain limited spectral regions, such as visible light, ultraviolet light or X-rays, for example, to obtain quantities like an X-ray luminosity. In radiometry, one would actually call such quantities the radiant flux or radiant power.
The initial letters “lumi” (from latin luminare = to illuminate) may seem to suggest that luminosity is a term of photometry (e.g. like luminous flux or luminance), where the spectral response of the human eye is taken into account. That is usually not the case, however – except in the context of luminosity functions, specifying the spectral sensitivity of the human eye.
A completely different meaning is used in the area of scattering theory and particle accelerators: there, the luminosity of a particle beam times an interaction cross section (e.g. of a bombarded particle) delivers the rate of interaction processes. A high luminosity of a particle accelerator – implying a small beam cross section and not only a high particle current – is often desired for investigating processes with small interaction cross sections.