noctilucent

noctilucent (nok-tuh-LOO-cuhnt) - adj., shining or glowing at night, especially of very high-altitude clouds that reflect sunlight long after sunset; nightshining.



Thanks, WikiMedia!

Also encompasses bioluminescent, but the clouds seem to be the primary meaning these days. Coined around 1890 from Latin roots that mean, literally, "night-shining."

---L.