The Grey Goo Scenario (also spelled differently as "Gray Goo Scenario") is a doomsday scenario popular in science-fiction but also theorized by some scientists, especially those who oppose nanotechnology.
This notion molecular nanotechnology in which out-of-control self-replicating robots consume all biomass on Earth while building more of themselves, a scenario that has been called ecophagy ("eating the environment", more literally "eating the habitation"). The original idea assumed machines were designed to have this capability, while popularizations have assumed that machines might somehow gain this capability by accident.
Self-replicating machines of the macroscopic variety were originally described by mathematician John von Neumann, and are sometimes referred to as von Neumann machines or clanking replicators.
The term gray goo was coined by nanotechnology pioneer Eric Drexler in his 1986 book Engines of Creation. In the year 2004, he stated, "I wish I had never used the term 'gray goo'." Engines of Creation mentions "gray goo" in two paragraphs and a note, while the popularized idea of gray goo was first publicized in a mass-circulation magazine, Omni, in November 22 years prior.
According to the basic Grey Goo Scenario, at some point in the future mankind (or other species) could create an advanced nanotechnological AI that would proceed to annex everything into itself, transforming the entire planet into "Grey/Gray Goo".