initial margin

In finance, a margin is collateral that the holder of a position in securities, options, or futures contracts has to deposit to cover the credit risk of his counterparty. 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, orentered 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 is formally called performance bond.
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Used in the context of general equities. 1) Amount of money deposited by both buyers and sellers of futures contracts to ensure performance of the terms of the contract; 2) amount of cash or eligible securities required to be deposited with a broker before engaging in margin  transactions.

A risk control measure that may be applied by the Eurosystem in reverse transactions, implying that the collateral required for a transaction is equal to the credit extended to the counterparty plus the value of the initial margin.
See also:
 margin call      variation margin (marking to market)    

When a customer establishes a position, he is required to make a minimum initial margin deposit to assure the performance of his obligations. Futures margin is earnest money or a performance bond.