It is from Cat's Cradle by Kurt Vonnegut. Basic story is that it is a form of ice that can freeze a room or higher temperature and when combined with other water it "teaches" the other water the form that allows it to freeze at room temperature. (It is like putting a crystal in a solution will sometimes lead to that crystal growing, but the solution wouldn't have formed into crystal without the first chunk being in it.) So the worry in the book is if this lab created ice gets into the wild, like into the ocean, it will teach the entire oceans the form and they will all freeze.
http://en.wikipedia.org/wiki/Ice-nine
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Wrong ice, I'm sure.