Self-replicating nanomachines spreading uncontrollably, building copies of themselves using all available material. Consists of a disassembler swarm combined with assembler units programmed to create more grey goo. Fortunately, efficient grey goo is very difficult to manufacture, because of energy constraints and elemental abundances.
The worst-case scenario where a grey-goo weapon is deployed is the complete conversion of a planet's biosphere (or sometimes, of its complete surface) into a single type of self-replicating unit. This scenario has rarely happened in reality, one of the few cases being the destruction of Makintairas during the Second Consolidation War.
In most cases grey goo forms part of a larger Khaki goo attack, incorporating many forms of self-replicating weapon. In most cases, modern grey and khaki goo attacks can be defeated rapidly by suitable blue goo defences.
Goo, Golden - Text by M. Alan Kazlev, from Anders Sandberg's Transhumanist Terminology A type of goo disaster, the strange name comes from an early Information Age hypothetical scenario. The idea was to use nanomachines to filter gold from seawater. If this process got out of control the result would be piles of golden goo (the " Wizard's Apprentice Problem"). This scenario demonstrated the need of keeping populations of self-replicating machines under control; it is much more common than grey goo, but also more manageable. Also Carbon Goo, Aluminium Goo, etc.
Goo, Green - Text by M. Alan Kazlev  A plague of bionano (non-drexlerian) goo; especially of strong biotic form. Green Goo outbreaks are (like Golden Goo) quite common, but easily managed, at least in those areas with good access to blue goo and equivalent.  Facetious term for biological life spreading throughout the galaxy. c.f. pink goo.  (archaic) The scenario of nanomachines or bio-engineered organisms used for human population control, either by governments or eco-terrorist groups (generally by sterilizing people through otherwise harmless infections).
Makintairas - Text by M. Alan Kazlev 10 Tauri - inner sphere system, destroyed by grey goo swarms during the Second Empires War - Metasoft was wrongly accused of the atrocity by a number of anthropist extremists and anti-vec propagandists of the time.