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.
Tactics for engaging cloaked UFOs are simple - don't use missiles, since they won't work. Instead, use beam weapons such as the Lineage Plasma Cannon and Medium Disruptor Beam.
See Alien Tech Levels for information on when Cloaking Fields will begin to be used.