Fama (Jan. 1965: ‘The behavior of stock-market prices’):
‘…an “efficient” market for securities, that is, a market where, given the available information, actual prices at every point in time represent very good estimates of intrinsic values.’

Fama (Sep.–Oct. 1965: ‘Random walks in stock market prices’):
‘An “efficient” market is defined as a market where there are large numbers of rational, profit-maximizers actively competing, with each trying to predict future market values of individual securities, and where important current information is almost freely available to all participants.’

Fama et al. (1969):
‘…an “efficient” market, i.e., a market that adjusts rapidly to new information.’

Fama (1970):
‘A market in which prices always “fully reflect” available information is called “efficient.”’

Jensen (1978):
‘A market is efficient with respect to information set θt if it is impossible to make economic profits by trading on the basis of information set θt.’
[‘By economic profits, we mean the risk adjusted returns net of all costs.’]

Fama (1991):
‘I take the market efficiency hypothesis to be the simple statement that security prices fully reflect all available information. A precondition for this strong version of the hypothesis is that information and trading costs, the costs of getting prices to reflect information, are always 0 (Grossman and Stiglitz (1980)). A weaker and economically more sensible version of the efficiency hypothesis says that prices reflect information to the point where the marginal benefits of acting on information (the profits to be made) do not exceed marginal costs (Jensen (1978)).’

Malkiel (1992):
‘A capital market is said to be efficient if it fully and correctly reflects all relevant information in determining security prices. Formally, the market is said to be efficient with respect to some information set, φ, if security prices would be unaffected by revealing that information to all participants. Moreover, efficiency with respect to an information set, φ, implies that it is impossible to make economic profits by trading on the basis of φ.’

Fama (1998):
‘…market efficiency (the hypothesis that prices fully reflect available information)...’
‘…the simple market efficiency story; that is, the expected value of abnormal returns is zero, but chance generates deviations from zero (anomalies) in both directions.’

Timmermann and Granger (2004):
‘A market is efficient with respect to the information set, Ωt, search technologies, St, and forecasting models, Mt, if it is impossible to make economic profits by trading on the basis of signals produced from a forecasting model in Mt defined over predictor variables in the information set Ωt and selected using a search technology in St.’