Efficient has a very specific meaning in economics (well, two specific meanings, depending on what kind market you're talking about).
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 includ...
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