A robot built on a molecular scale.
Found in copious quantities in science fiction where they exhibit properties such as rapidly replicating regardless of available suitable molecular feedstock, curing tinea, decapitation and stomach ulcers and repairing plot holes.
Also found in small numbers in scientific literature where they show great promise in doing all kinds of really freaking cool stuff, once they grow up a bit.
2. "You were caught in an atomic explosion but we managed to reconstruct you with the help of nanobots."
3. "The nanobots are eating my clothing!"
4. "Initial atomic force microscopy of the nanobots indicates a high level of freaking coolness"