Ever since the late 1990's - when technology stocks were vastly overpriced (remember when Cisco was going to be the first company worth $1 trillion?) - analysts and commentators have become very quick to say a particular market or asset is in a vastly overpriced "bubble".
Occasionally these bubble comments are right (e.g. Las Vegas real estate in the middle of the last decade) but I often think they are missing the point. To me, a true asset price bubble is one which is a longer term phenomena where prices are vastly higher than the economic worth of a particular security or property.
Robert Shiller of Yale (he of Irrational Exuberance fame) gave a talk in Davos yesterday where he identified the characteristics of a bubble (reprinted from the New York Times blog):