An ansible is a device that allows for faster-than-light communication.
How It Works
The most common variety of ansible works by creating an extremely skinny wormhole that is just wide enough to allow for photons to pass through. Because such a wormhole is orders of magnitudes longer than it is wide, it effectively produces the Casmir effect of negative energy that allows the wormhole to remain open for a long enough time to send a pulse of light through.
http://www.newscientist.com/article/dn25596#.U_J_MfldXE0
Early versions of the ansible can only connect two locations in spacetime together, and are built in pairs that can only communicate with each other. More advanced versions of the ansible are capable of opening a channel to any other properly addressed open ansible.