An emission nebula was a type of nebula that emitted its own light, due to the presence of ultraviolet radiation from very hot young stars inside. Cloaking devices were useless inside emission nebulae.
The Cardassian Amleth system was 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")