Noctilucent is an adjective that describes something which is visible or shines in the dark. It comes from two Latin words, “nox”, meaning night and “lux” meaning light. So literally it means ‘shining in the night’. This term is often used to describe a type of cloud that appears in the Earth's sky during twilight hours, when the sun is very low on the horizon. These clouds are made up of tiny ice crystals and can be seen glowing brightly against the dark background of the night sky.