Efficient Market

A market where prices reflect all available information.

The efficient-market hypothesis (EMH)[a] is a hypothesis in financial economics that states that asset prices reflect all available information. A direct implication is that it is impossible to "beat the market" consistently on a risk-adjusted basis since market prices should only react to new information.
Because the EMH is formulated in terms of risk adjustment, it only makes testable predictions when coupled with a particular model of risk.[2] As a result, research in financial economics since at least the 1990s has focused on market anomalies, that is, deviations from specific models of risk.[3].
The idea that financial market returns are difficult to predict goes back to Bachelier,[4] Benoit Mandelbrot,[5] and Paul Samuelson,[6] but is closely associated with Eugene Fama, in part due to his influential 1970 review of the theoretical and empirical research.[2] The EMH provides the basic logic for modern risk-based theories of asset prices, and frameworks such as consumption-based asset pricing and intermediary asset pricing can be thought of as the combination of a model of risk with the EMH. https://en.wikipedia.org/wiki/Efficient-market_hypothesis

• The efficient markets theory was not popular until the 1960s when the advent of computers made it possible to compare calculations and prices of hundreds of stocks more quickly and effortlessly. In 1945, F.A. Hayek argued in his article The Use of Knowledge in Society that markets were the most effective way of aggregating the pieces of information dispersed among individuals within a society. Given the ability to profit from private information, self-interested traders are motivated to acquire and act on their private information. In doing so, traders contribute to more and more efficient market prices. In the competitive limit, market prices reflect all available information and prices can only move in response to news. Thus there is a very close link between EMH and the random walk hypothesis.
• Investors, including the likes of Warren Buffett,[31] George Soros,[32][33] and researchers have disputed the efficient-market hypothesis both empirically and theoretically. Behavioral economists attribute the imperfections in financial markets to a combination of cognitive biases such as overconfidence, overreaction, representative bias, information bias, and various other predictable human errors in reasoning and information processing. These have been researched by psychologists such as Daniel Kahneman, Amos Tversky and Paul Slovic and economist Richard Thaler.