A cloaking device is a form of stealth technology that uses selective blending of light (and other forms of energy) to render a starship or other object completely invisible to the electromagnetic spectrum and most sensors. It has been encountered in varying forms over the centuries.
Why do ships in Horizon Fleet have cloaking devices?
The Treaty of Algeron was a peace treaty signed between the United Federation of Planets and the Romulan Star Empire in 2311 following the events of the Tomed Incident. The treaty reinforced and redefined the Romulan Neutral Zone and made clear that any violations of the Zone without adequate notification by either side would be considered an act of war. The treaty also expressly prohibited the development or use of cloaking device technology by the Federation.
The most notable violation occurred in 2358 when a select group within Starfleet Intelligence developed a cloaking device which also enabled a vessel to pass through any 'normal' matter such as a planet or asteroid. They installed the phase-cloak aboard the Orbeth-class USS Pegasus for field testing under the supervision of then-Captain Erik Pressman.
An exception to the treaty in 2371 allowed for the limited supervised use of at least one cloaking device on loan from the Romulans. Operation of the cloaking device was to be limited to the Gamma Quadrant. In return, the Romulans would receive all information the Federation obtained on the USS Defiant's voyages into the Gamma Quadrant, most importantly any information concerning the Dominion. As such, the Defiant was the only Federation ship to be permanently equipped with a cloaking device until the mid 2380s.
The Romulan Star Empire entered into a second joint intelligence gathering operation with Starfleet following first contact with the Elyshan Empire in 2384. Rumors that Elyshan vessels were equipped with subspace weapons arrays, subspace shielding systems, and subspace cloaking technology elicited much concern from the Federation Council as well as the Romulan Senate. By the year 2388, the Federation had renegotiated the original amendment to include operations against the Elyshan Empire and the Zutal Imperium. Installation of Romulan-supplied cloaking devices was limited to a select number of starships operating along the Federation core-ward frontier.