A cloaking device bends light and energy around a vessel to make it invisible to sight and sensors, a technology long associated with Romulan and Klingon ships.
One of Starfleet's most famous early encounters with cloaking technology came in 2266, when a Romulan Bird-of-Prey used one to destroy Federation outposts along the Neutral Zone. Captain Kirk later stole a Romulan cloaking device during a covert mission. Klingon ships adopted cloaks as well, and the Bird-of-Prey became the classic cloaked raider.
Early cloaked ships had to decloak before firing, but a prototype Klingon Bird-of-Prey commanded by General Chang was able to fire torpedoes while still cloaked, nearly defeating the Enterprise-A and Excelsior until it was tracked by its exhaust.
The Treaty of Algeron barred the Federation from developing cloaking technology. In the 2370s the Romulans loaned a cloaking device to the USS Defiant for use in the Gamma Quadrant, operated under strict conditions. Starfleet nevertheless worked on detection methods, such as tachyon networks, to locate cloaked ships.