An ansible is a fictitious machine capable of instantaneous or superluminal communication. typically it is depicted as a lunch box-sized object with some combination of microphone, speaker, keyboard and display. It can send and receive messages to and from a corresponding device over any distance whatsoever with no delay. Urusula Le Guin states that she derived the name from "answerable," as the device would allow its users to receive answers to their messages in a reasonable amount of time, even over interstellar distances.

Variations on the device from other science fiction worlds include: a sympathetically vibrating Black Crystal from The Crystal Singer, aetherschreibers  from The Age of Unreason, Symbiotic Crystalline Resonance Transmissions from The Gap CycleCausal channels which use entangled particles for instantaneous two-way communication from Stross's Singularity SkyHyperspatial needlecast, a technology infinitesimally close to instantaneous to the point of being immeasurable, rhodomagnetic waves hyper-wave relaybipolar mathematics or ancient communication stones.