So'wI' yIchu' 
Engage the cloaking device! TKD/ST3/ST5/ST6

So'wI' yIchu'Ha' 
Disengage cloaking device! CK/ST3

So'wI' vIchu'ta' 
I have engaged the cloaking device. TKD

So'wI' chu'lu'ta' 
The cloaking device has been engaged.TKD

So'wI' chu'ta' 
Cloaking device engaged! (Clipped) TKD

Hoch tlhIngan DujDaq So'wI' jomlu'. puvtaHbogh Duj ngabmoHlaw' So'wI' 
All Klingon vessels are equipped with a cloaking device allowing the ship to fly in a state of practical invisibility. S33

boq lucherDI' tlhIngan wo' romuluS Hov wo' je So'wI' cham Suqpu' tlhIngan wo'.
Cloaking technology was gained through an alliance with the Romulan Star Empire... S33

HoS law'qu' natlhmo' So'wI' QaptaHvIS So'wI' QaplaHbe' nuHmey 
Due to the tremendous energy drain of a cloaking device, weapons cannot be discharged while the cloak is in operation. S33

ngongmeH wa' DujDaq nuHmey nISbe'bogh So'wI' jomlu'pu' 
[A cloaking device which didn't disrupt weaponry was installed in one experimental ship. (untranslated on card)] S33

QaptaHvIS So'wI', Hoch jagh Dujmey DaQotlh.
Disable all enemy ships while cloaked. MKE

 QapHa' So'wI' vaj Datu'lu'
Cloaking device fails and you are discovered. MKE

Non-canon:

Duj jun yu'eghmey tIHmey je 'e' qaSmoHbogh tlhamchem tIn chenmoH tlhIngan So'wI' 'e' wI'aq. 

We suspect the Klingon cloak generates a massive gravitational field that bends light and other electromagnetic waves around the ship. (DSC/subtitle "Into the Forest I Go") 

(TKD 20) : [So'wI'] comes from the verb So' "cloak" plus -wI' "thing which does". So'wI' is a "thing which cloaks." 

(KGT 54f):  Perhaps the most characteristic of the modern Klingon armaments--though, strictly speaking, not an armament at all--is the cloaking device (So'wI'). Most, if not all, Klingon ships are equipped with such a device that makes it all but impossible for them to be detected by enemies' sensors. The workings of the So'wI' are apparently based on the Romulan cloaking device, the Klingons having learned (some would say `stolen') the technology during their brief alliance with the Romulan Empire in the mid-twenty-third century. The So'wI' is not perfect, however. For one thing, ways have been developed to detect a cloaked ship, though these techniques are difficult and may not work quickly enough to allow an appropriate reaction. The device also requires so much of the ship's energy to operate that it is not possible to fire weapons while the ship is cloaked. A cloaking device that did allow weapons to be fired while the ship was cloaked was developed by Klingon scientists and actually installed on one ship, but that ship was detected anyway and further development of the improved technology seems to have been put on hold. In this case, traditional Klingon values seem to have prevailed over potential tactical advantage. From the Klingon point of view, firing while cloaked, and therefore being unseen and unknown by the enemy, would have no honor. The So'wI' that must be deactivated before firing weapons remains a standard part of a ship's equipment. 

    The terminology associated with using a cloaking device is simply chu' (activate) and chu'Ha' (deactivate). It is also appropriate to say Duj So' ("He/she cloaks the ship") or Duj So'lu'pu' ("The ship is cloaked"; literally, "Someone or something has cloaked the ship").

SkyBox S33:  All Klingon vessels are equipped with a cloaking device, allowing the ship to fly in a state of practical invisibility. Cloaking technology was gained through an alliance with the Romulan Star Empire, possibly in exchange for Warp Drive technology. Due to the tremendous energy drain of a cloaking device, weapons cannot be discharged while the cloak is in operation. [A cloaking device which didn't disrupt weaponry was installed in one experimental ship. On Stardate 9521.6 this ship was demonstrated but it was destroyed.]" 

The stolen Romulan cloaking device had to be hooked into the deflector shield control on the Enterprise. (TOS "The Enterprise Incident")]

SEE:

So'             hide,  conceal,  cloak,  encrypt (v)
So'Ha'                  uncloak,  decrypt (v)
So'wI' DaH      cloak array (n)