Luminosity is the quality of something that gives off light or shines with reflected light. The most noticeable quality of a large, sparkly diamond is its luminosity.
You might describe a bright and lively oil painting in terms of its luminosity, or marvel at the luminosity of a brilliant sunset. Astronomers use the word luminosity to talk about a specific property of physics, the energy given of by an astronomical object in a certain amount of time. There's a direct correlation between this amount of energy and the object's brightness. The Latin root is lumen, meaning "light."