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In finance, a long position indicates that the investor/trader promises to purchase an asset in the future. As a result, an increase in the asset price creates a gain for the holder of a long position contract.
In the derivatives market, a long position implies that the holder of a long position contract promises to purchase the asset at a prespecified price for the delivery of the asset at a future date. For example, a trader taking a long position in a commodity futures contract promises to purchase the commodity at the delivery date by paying the prespecified future price at the delivery date.
Similarly, a long position in a call option for a foreign currency indicates that the holder of the option contract will take delivery of the foreign currency at the maturity (or perhaps before maturity depending on the type of option contract).
To sum up, one of the parties to a contract involving a derivative assumes a long position and commits to buying the underlying asset/instrument on a certain future delivery date for an agreed-upon price. The other party takes a short position and commits to selling the same asset/instrument on the same delivery date for the same agreed-upon price.