An economic theory related to Finance and it states that share prices or the assets defines all the available information. This implicates that one cannot "outperform the market" continuously unless there's risk adjustments have been made because the stocks are always traded at fair prices. These implications make difficulties for the investors while buying and selling their assets. So investor can only be successful in getting higher values either by chance or by taking risks.
Diagrams
Originated from/Coined by
Eugene Fama
Further Reading/ References
https://www.investopedia.com/terms/e/efficientmarkethypothesis.asp