The efficient market hypothesis is the statement that asset prices are always correct, in the sense that they never make predictable updates (change in a way predictable in advance from information available in the past). As a corollary, as prices are now attached to a very wide variety of phenomena (weather, personal insurance, long-dated options, etc), the market is nigh-omniscient. The controversial strong-form EMH states additionally that assets are always priced correctly, and thus that people always make decisions which are in their self-interest.
A screaming bird of truth.