Mask Energy Signature is the science ability that can "cloak" a ship that does not have a cloaking device. It decreases your EM signature, and darkens your ships visual appearance. Not a cool as a cloaking device, but it can keep you from being detected by NPCs most times. Players do still tend to "see" you, and if they "scan" can target you. So other then having a Klingon ship, or one of the few Fed ships, with a cloaking device your going to use Mask Energy Signature.
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There is a list here
http://www.stowiki.org/Cloak
For a Klingon ship, THIS is the button