Definitions for "Efficient Market Hypothesis"
A hypothesis that U.S. equity markets are efficient. Bloopers & Blunders: Reasoning for the Surge in the Dow. Finance By Example (Archives): Bubbles in History: Is There Something To Be Learned? Evidence: Few Brokerage Firms Beat the Market Last Year The Stampede To Index Funds Internet Chat and Market Efficiency Two Analysts’ Forecasts Diverge by 2,000 Points! Humor
An asset market is information-efficient if asset prices fully reflect all currently available information on future asset returns.  New information is incorporated quickly and correctly into asset prices through market trading.
the idea that markets adjust rapidly enough to eliminate profit opportunities immediately.