Market timing is the strategy of making buying or selling decisions of financial assets (often stocks) by attempting to predict future market price movements. The prediction may be based on an outlook of market or economic conditions resulting from technical or fundamental analysis. This is an investment strategy based on the outlook for an aggregate market rather than for a particular financial asset.
The efficient-market hypothesis is an assumption that asset prices reflect all available information, meaning that it is theoretically impossible to systematically "beat the market."