"Irrational exuberance" is a phrase used by Federal Reserve Board Chairman Alan Greenspan in a speech given during the stock market boom of the 1990s. The phrase was interpreted by financial pundits as a typically cryptic warning that the market might be overvalued.
Although it is sometimes believed that Greenspan's comment was made near the height of the dot-com boom (and contributed to its downfall), it was actually said much earlier, in a December 1996 dinner lecture to the American Enterprise Institute in Washington