A Wraith in the process of cloaking itself.
The Wraith Cloaking Device enables the Wraith to cloak. The Wraith have an instictual ability to cloak. Much in the same way a human will put on a coat for certain situatuions, a Wraith will be able to cloak. It is a fairly big mystery how this occurs, but needless to say, at one moment the Wraith will be there and the next it won't. It is believed that the Wraith fold space and time onto themselves to achieve this effect. It is the subject of a few missions in the Polaris storyline, and those missions end in the availability of the Zephyr, Tunneling Organ, and the Polaris Cloaking Organ.