A cloaking device was a type of energy projector that could be used to render a starship or other object invisible and completely undetectable to most sensors. The Romulans are considered to be one of the first major powers in the Alpha or Beta Quadrant to develop a cloaking device. The Klingons, by virtue of an alliance with the Romulans in 2268 which resulted in an exchange of technologies and spacecraft designs, also now commonly use cloaking devices on most of their vessels.
The Federation is banned by the Treaty of Algeron from developing or using cloaking device technology in its spacecraft.