You may want to dig up Fama's work from the 1960s, which underlies the modern approach to the EMH. If you're courageous, try Bachelier's 1900 doctoral dissertation, "The Theory of Speculation" -- substantially more interesting than latter-day, textbook regurgitations.
An 'efficient' market is defined as a market where there are large numbers of rational, profit-maximizers actively competing, with each trying to predict future market values of individual securities, and where important current information is almost freely available to all participants. In an efficient market, competition among the many intelligent participants leads to a situation where, at any point in time, actual prices of individual securities already reflect the effects of information based both on events that have already occurred and on events which, as of now, the market expects to take place in the future. In other words, in an efficient market at any point in time the actual price of a security will be a good estimate of its intrinsic value.
Eugene F. Fama, "Random Walks in Stock Market Prices," Financial Analysts Journal, September/October 1965 (reprinted January-February 1995).
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