gray goo n.
A hypothetical substance composed of sagans 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.