The "strong form" of market efficiency is a theory in finance that suggests all information, both public and private, is fully reflected in asset prices. This means that no investor can achieve consistently higher returns than the overall market, even with insider information. In a strong form efficient market, even those with access to confidential information cannot outperform the market average. This concept is part of the broader Efficient Market Hypothesis (EMH), which also includes weak and semi-strong forms, each addressing different types of information and their impact on stock prices.