Astronomy and Physics. The intensity of light observed from a light source, esp. a star or other celestial object (cf. magnitude, n. 3a). Also…

The property of naturally producing and emitting light, as distinguished from reflecting light.

Optics. Degree of lighting up; the intensity of the light falling upon a surface, as measurable by the amount incident on each unit of the surface.

The effectiveness of light of any particular wavelength in producing the sensation of brightness when perceived.

The amount of luminous flux per unit area; = illumination, n. 1b.

Physics. The amount of luminous flux emitted by unit area of a source into unit solid angle (the objective analogue of subjective brightness).

The flux of radiant energy per unit area, esp. an area normal to the direction of travel through a medium.