Luminous means giving off light or glowing brightly. A luminous watch dial glows in the dark so you can read the time at night. Fireflies are luminous insects, creating their own light through a chemical reaction in their bodies. The moon appears luminous in the night sky, though it's actually reflecting sunlight rather than producing its own light.

Scientists use luminous to describe anything that emits light, whether it's a star, a light bulb, or certain deep-sea creatures that glow in the ocean's darkness. The word can also describe something that seems to glow with reflected light, like luminous eyes that catch the light just right, or a luminous pearl that seems to shine from within.

Beyond physical light, people sometimes describe faces or expressions as luminous when they seem to radiate joy, intelligence, or emotion. When someone's face becomes luminous with happiness, it means their delight is so obvious it's almost like they're glowing. A luminous smile might light up an entire room, not because it literally produces light, but because it's so warm and bright it affects everyone around it.