Cloaking Field (Apocalypse)
The cloaking field is a large alien device found on some UFOs that provides a craft with protection from missiles. Specifically, while a craft is "cloaked" (visible as a partial-transparency effect), missiles targetting it will not turn. The "cloaking" effect drops for a short time whenever the cloaked craft fires weapons.
While UFO types 6, 7 and 8 make good use of the Cloaking Field against X-COM, the Cloaking Field's unwieldy size of 3x4 and its research prerequisite of the Large Disruption Shield make it rather difficult for X-COM to employ the device. Only the Hawk Air Warrior, Bio-Trans, Retaliator and Annihilator can equip a Cloaking Field, and all must give up 800 points worth of shielding for the privilege. Moreover, a typical fighter loadout of multiple beam weapons will render the cloak nearly useless, as the rapid fire rate of these weapons will prevent the cloak from re-engaging. Cloaking an unarmed Bio-Trans isn't the worst of ideas, but otherwise, shielding is generally superior.
See Alien Tech Levels for information on when Cloaking Fields will begin to be used.