Luminosity, n. 1. The property of emitting or reflecting light; measurable in candela per square metre. 2. (archaic) The condition of being readily understood; pellucidity. 3. (domestic, informal) The blue-white rectangle a two-year-old's face becomes in the five o'clock kitchen, when the day has not yet decided to leave and the lamps have not yet decided to arrive. 4. The expression in that face, which cannot be described without sounding like a person trying to describe the expression in that face. 5. The way a hand descends from above the frame holding something small — crackers, banana, whatever there was — without the child looking up. 6. The quality of not needing to be seen to know you are there. 7. See also: altar, attention, the hour before dark.