Minimum Margin
The initial amount required to be deposited in a margin account before trading on margin or selling short. For example, the NYSE and the NASD require investors to deposit a minimum of $2,000 in cash or securities to open a margin account. Keep in mind that this amount is only a minimum - some brokerages may require you to deposit more than $2,000.
Investopedia Commentary
When you buy on margin, there are key levels - as governed by the Federal Reserve Board's Regulation T - that must be maintained throughout the life of a trade. The minimum margin, which states that a broker can't extend any credit to accounts with less than $2,000 in cash (or securities) is the first requirement. Second, an initial margin of 50% is required for a trade to be entered. Third, the maintenance margin says that you must maintain equity of at least 25% or be hit with a margin call.
Related Links
Margin Trading Tutorial
See also: Broker's Call, Buying Power, Equity, Federal Reserve Board - FRB, Initial Margin, Leverage, Maintenance Margin, Margin Account, Margin Call, Regulation T