Efficient Capital Markets
Markets where share prices accurately reflect all currently available information that has bearing on the intrinsic value of the underlying enterprise. If markets are really like this then it would be impossible to ‘beat the market’ without insider information. Whether or not capital markets really are efficient is a matter of considerable debate.
The idea that capital markets are efficient appears to be disproved by stock market bubbles such as the one in IT stocks during the late 1990s. In bidding up the prices of stocks in companies that had no earnings and no proven track record, the market acted in way that was neither rational nor efficient.