What is gray goo?
"Gray goo" is a term used to describe what might happen if teeny-tiny robots were created with the ability—and imperative—to use the substances around them to create more, identical teeny-tiny robots. The idea is that this process could get out of control, with a wave of teeny-tiny robots overrunning the earth and turning everything to gray goo.
The term is also used in the online virtual world of Second Life for self-replicating objects that get out of hand in a similar manner, to the point where they crash the computer server on which they're running.
—The Fact Monster