Cloak

From The StarCraft 2 Wiki

Cloaking is a passive & active unit ability that some units and buildings of StarCraft and StarCraft II can have, or be given, and has the basic function of making it invisible to any unit that does not have a detector nearby. Being cloaked will not give the unit/building the ability to see other cloaked units. Some units have cloaking at all times, like Protoss Observers, and some need to activate it, like Terran Ghosts. There are also units who can give cloaking to other units/buildings.