Efficient Market Hypothesis (EMH)

The academic theory, championed by economist Eugene Fama, that asset prices fully reflect all available information at any given time, making it impossible to consistently "beat the market" through stock picking or market timing. EMH comes in three forms -- weak, semi-strong, and strong -- depending on what type of information is considered priced in. The existence of traders like Warren Buffett and Jim Simons who have outperformed for decades keeps the debate very much alive.