What is Uncovered Interest Arbitrage?

Legal Definition
Uncovered interest arbitrage is an arbitrage trading strategy whereby an investor capitalizes on the interest rate differential between two countries. Unlike covered interest arbitrage, uncovered interest arbitrage involves no hedging of foreign exchange risk with the use of forward contracts or any other contract. The strategy involves risk, as an investor exposed to exchange rate fluctuations is speculating that exchange rates will remain favorable enough for arbitrage to be profitable. The opportunity to earn profits arises from the reality that the uncovered interest rate parity condition does not constantly hold—that is, the interest rate on investments in one country's currency does not always equal the interest rate on foreign-currency investments plus the rate of appreciation that is expected for the foreign currency relative to the domestic currency. When a discrepancy between these occurs, investors who are willing to take on risk will not be indifferent between the two possible locations of investment, and will invest in whichever currency is expected to offer a higher rate of return including currency exchange gains or losses (perhaps adjusted for a risk premium).
-- Wikipedia