The ansible is a device in a number of works of science fiction, the first of which was Ursula K. Le Guin's Rocannon's World, and the most famous of which is Orson Scott Card's Ender series. In the Ender series, the official name of the device is the 'Philotic Parallax Instantaneous Communicator', and it is further explained that the term 'ansible' was "dredged out of an old book somewhere," a definite shout-out to LeGuin.
The Card ansible enables faster-than-light transfer of information in a universe where faster-than-light movement of matter and energy is impossible. The way that the ansible works is somewhat analogous to quantum entanglement: two quarks in a pi meson are separated and contained within the ansible. The two quarks, despite being physically separated, remain connected to one another through what Card calls a 'philotic ray'. Transmission of information takes place through manipulating one of the quarks, which reverberates along the philotic ray and affects the other quark instantaneously, no matter the distance.