gray goo
n.
A hypothetical substance composed of {sagan}s of sub-micron-sized
self-replicating robots programmed to make copies of themselves out of
whatever is available. The image that goes with the term is one of the
entire biosphere of Earth being eventually converted to robot goo.
This is the simplest of the {nanotechnology} disaster scenarios,
easily refuted by arguments from energy requirements and elemental
abundances. Compare {blue goo}.