OK this isn't exactly a new word, I've come across it many times without ever being sufficiently bothered to look up its exact meaning. That's often an acceptable modus operandi when you can make a believable (if only to you) guess at a word's meaning, but that's something I never really managed to do with "externalities" — the best I managed was that it referred to something nasty.
So here's a definition from Paul Krugman, no less, in a recent article in the New York Times: "externalities — costs that people impose on others without paying the price, like traffic congestion or pollution." So now I know.
