"Grey goo" is a term coined by Eric Drexler in his 1986 book about nanotechnology, Engines of Creation. In one section of the book, Drexler speculates about what could happen if we created self-reproducing "nanobots." If they got out of control, they could conceivably use the resources of the entire planet to replicate in an exponential manner akin to bacterial replication. Every living creature, every useful mineral, would be converted into more of the "gray goo."
Like it or not, science fiction is becoming science fact.
The Terminator was, of course, a work of fiction, but the idea of cyborgs and killer machines is not all that farfetched. In fact, the danger is becoming so real that the Obama Administration found the need to spell out explicit rules that specify under what circumstances machines are allowed to kill humans.