请教Margin在美国财务上的准确意义

　　请看英文解释以便得到准确意义：

　　In finance, a margin is a collateral that the holder of a position in securities, options, or futures contracts has to deposit to cover the credit risk of his counterparty (most often his broker). This risk can arise if the holder has done any of the following:

　　- borrowed cash from the counterparty to buy securities or options,
　　- sold securities or options short, or
　　- entered into a futures contract.

　　The collateral can be in the form of cash or securities, and it is deposited in a margin account. On U.S. futures exchanges, "margin" was formally called performance bond.

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　　Definition 1
　　Banking: (1) Difference between the market value of a collateral and amount of the loan advanced against it. Also called haircut. (2) Percentage added to a market rate of interest, or subtracted from a market rate of deposit, to provide a return to the b