means The quantity or quality of light a thing gives off — in astronomy, the total energy a star or other object radiates into space per second.

from From Latin 'lumen' (light), which gives us 'luminous' and a whole glowing family of words. The noun 'luminosity' arrives through the Latin 'luminositas', built on the same root, and entered English by the 17th century as a learned term for brightness before astronomers borrowed it to measure how fiercely a star burns.