A planet outside our solar system that is approximately the same size as the Earth and roughly the same distance from its star.
Slightly smaller than Earth, KOI 326.01 lies in the habitable zone of its star, a red dwarf star dimmer than our sun. At around 100 light years away, it's "fairly close" to us, said William Borucki, of NASA's Ames Research Center, "but they're not places you can walk to on a Sunday".
Currently just a promising data point, KOI 326.01 (which stands for Kepler Object of Interest) might end up getting a better name if it does turn out to be a real Earth twin.
—Rowan Hooper, "Exoplanet findings spark philosophical debate," New Scientist, February 21, 2011
The "Earth twin distance" is how far a planet would have to be from the star to receive the same insolation that Earth receives from the Sun.
—George Ochoa and Jeffrey Osier, "The Writer's Guide to Creating a Science Fiction Universe," Writer's Digest Books, March 1,1993