Cloak
From ASSS Wiki
A ship is said to be cloaking if it cannot be seen on screen by its enemies. It will still appear on radar (unless they are also using Stealth). Ships using X-Radar can see ships that are using Cloak and Stealth. Using Cloak usually slows down how fast a ship recharges, depending on the Settings.
After firing weapons your ship will automatically uncloak. The exception to this is dropping a brick or portal.