A term describing what remains on the planet as a result of a science fiction nightmare. Gray goo comes from the 1986 book "Engines of Creation, The Coming Era of Nanotechnology" by K. Eric Drexler.
Nanomachines, called "assemblers" were created from organic molecules that were designed to replicate themselves. However, if something went amiss, they could replicate until the planet is void of all organic matter and there is nothing left but "gray goo." See
nanofactory.