An emission nebula is a type of nebula that emits its own light, due to the presence of ultraviolet radiation from very hot young stars inside. Cloaking devices are useless inside emission nebulae.
The Cardassian Amleth system is located within an emission nebula. (DS9: "Return to Grace")
In 2372, Tom Paris suggested that the USS Voyager travel through an emission nebula to save time. However, Chakotay rejected his proposal. (VOY: "Lifesigns")