The three you mention are all subtypes of the same efficiency—informational efficiency. Informational efficiency is used in finance and refers to how well a financial market incorporates information into prices. Basically a market is informationally efficient if you can’t out-predict without using information it doesn’t have. The weak / semi-strong / strong distinction merely indicates how much information it is incorporating into prices: weak means it’s incorporating it’s own past prices, semi-strong includes all public information, and strong includes all information held in private as well.
The other type of efficiency is allocative efficiency, a concept used in microeconomics. An allocatively efficient market is one that assigns goods to the people who place the highest value on them (subject to the constraints of each person’s endowments). It is effectively a utility-maximising condition. The whole concept of market failure in economics is built around situations where markets are failing to be allocatively efficient.
I know of three, although it is a matter of parametrization (weak, strong, semi-strong). What two meanings do you have in mind?
The three you mention are all subtypes of the same efficiency—informational efficiency. Informational efficiency is used in finance and refers to how well a financial market incorporates information into prices. Basically a market is informationally efficient if you can’t out-predict without using information it doesn’t have. The weak / semi-strong / strong distinction merely indicates how much information it is incorporating into prices: weak means it’s incorporating it’s own past prices, semi-strong includes all public information, and strong includes all information held in private as well.
The other type of efficiency is allocative efficiency, a concept used in microeconomics. An allocatively efficient market is one that assigns goods to the people who place the highest value on them (subject to the constraints of each person’s endowments). It is effectively a utility-maximising condition. The whole concept of market failure in economics is built around situations where markets are failing to be allocatively efficient.